Boost C++ Libraries

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Advanced Topics

Default Parameters
Commas and Symbols in Macros
Assignments and Returns
Nesting
Accessing Types (concepts, etc)
Specifying Types (no Boost.Typeof)
Inlining
Recursion
Overloading
Exception Specifications
Storage Classifiers
Same Line Expansions
Limitations (operators, etc)

This section illustrates advanced usage of this library. At the bottom there is also a list of known limitations of this library.

This library allows to specify default values for the local function parameters. However, the usual C++ syntax for default parameters that uses the assignment symbol = cannot be used. [17] The keyword default is used instead:

parameter-type parameter-name, default parameter-default-value, ...

For example, let's program a local function add(x, y) where the second parameter y is optional and has a default value of 2 (see also add_default.cpp):

int BOOST_LOCAL_FUNCTION(int x, int y, default 2) { // Default parameter.
    return x + y;
} BOOST_LOCAL_FUNCTION_NAME(add)

BOOST_TEST(add(1) == 3);

Programmers can define a WITH_DEFAULT macro similar to the following if they think it improves readability over the above syntax (see also add_with_default.cpp): [18]

#define WITH_DEFAULT , default

int BOOST_LOCAL_FUNCTION(int x, int y WITH_DEFAULT 2) { // Default.
    return x + y;
} BOOST_LOCAL_FUNCTION_NAME(add)

BOOST_TEST(add(1) == 3);

The C++ preprocessor does not allow commas , within macro parameters unless they are wrapped by round parenthesis () (see the Boost.Utility/IdentityType documentation for details). Therefore, using commas within local function parameters and bindings will generate (cryptic) preprocessor errors unless they are wrapped with an extra set of round parenthesis () as explained here.

[Note] Note

Also macro parameters with commas wrapped by angular parenthesis <> (templates, etc) or square parenthesis [] (multidimensional array access, etc) need to be wrapped by the extra round parenthesis () as explained here (this is because the preprocessor only recognizes the round parenthesis and it does not recognize angular, square, or any other type of parenthesis). However, macro parameters with commas which are already wrapped by round parenthesis () are fine (function calls, some value expressions, etc).

In addition, local function parameter types cannot start with non-alphanumeric symbols (alphanumeric symbols are A-Z, a-z, and 0-9). [19] The library will generate (cryptic) preprocessor errors if a parameter type starts with a non-alphanumeric symbol.

Let's consider the following example:

void BOOST_LOCAL_FUNCTION(
    const std::map<std::string, size_t>& m,                 // (1) Error.
    ::sign_t sign,                                          // (2) Error.
    const size_t& factor,
            default key_sizeof<std::string, size_t>::value, // (3) Error.
    const std::string& separator, default cat(":", " ")     // (4) OK.
) {
    ...
} BOOST_LOCAL_FUNCTION_NAME(f)

(1) The parameter type const std::map<std::string, size_t>& contains a comma , after the first template parameter std::string. This comma is not wrapped by any round parenthesis () thus it will cause a preprocessor error. [20] The Boost.Utility/IdentityType macro BOOST_IDENTITY_TYPE((type-with-commas)) defined in the boost/utility/identity_type.hpp header can be used to wrap a type within extra parenthesis () so to overcome this problem:

#include <boost/utility/identity_type.hpp>

void BOOST_LOCAL_FUNCTION(
    BOOST_IDENTITY_TYPE((const std::map<std::string, size_t>&)) m, // OK.
    ...
) {
    ...
} BOOST_LOCAL_FUNCTION_NAME(f)

This macro expands to an expression that evaluates (at compile-time) exactly to the specified type (furthermore, this macro does not use variadic macros so it works on any C++03 compiler). Note that a total of two set of parenthesis () are needed: The parenthesis to invoke the BOOST_IDENTITY_TYPE(...) macro plus the parenthesis to wrap the type expression (and therefore any comma , that it contains) passed as parameter to the BOOST_IDENTITY_TYPE((...)) macro. Finally, the BOOST_IDENTITY_TYPE macro must be prefixed by the typename keyword typename BOOST_IDENTITY_TYPE(parenthesized-type) when used together with the BOOST_LOCAL_FUNCTION_TPL macro within templates.

[Note] Note

Often, there might be better ways to overcome this limitation that lead to code which is more readable than the one using the BOOST_IDENTITY_TYPE macro.

For example, in this case a typedef from the enclosing scope could have been used to obtain the following valid and perhaps more readable code:

typedef std::map<std::string, size_t> map_type;
void BOOST_LOCAL_FUNCTION(
    const map_type& m, // OK (and more readable).
    ...
) BOOST_LOCAL_FUNCTION_NAME(f)

(2) The parameter type ::sign_t starts with the non-alphanumeric symbols :: thus it will generate preprocessor errors if used as a local function parameter type. The BOOST_IDENTITY_TYPE macro can also be used to overcome this issue:

void BOOST_LOCAL_FUNCTION(
    ...
    BOOST_IDENTITY_TYPE((::sign_t)) sign, // OK.
    ...
) {
    ...
} BOOST_LOCAL_FUNCTION_NAME(f)
[Note] Note

Often, there might be better ways to overcome this limitation that lead to code which is more readable than the one using the BOOST_IDENTITY_TYPE macro.

For example, in this case the symbols :: could have been simply dropped to obtain the following valid and perhaps more readable code:

void BOOST_LOCAL_FUNCTION(
    ...
    sign_t sign, // OK (and more readable).
    ...
) {
    ...
} BOOST_LOCAL_FUNCTION_NAME(f)

(3) The default parameter value key_sizeof<std::string, size_t>::value contains a comma , after the first template parameter std::string. Again, this comma is not wrapped by any parenthesis () so it will cause a preprocessor error. Because this is a value expression (and not a type expression), it can simply be wrapped within an extra set of round parenthesis ():

void BOOST_LOCAL_FUNCTION(
    ...
    const size_t& factor,
            default (key_sizeof<std::string, size_t>::value), // OK.
    ...
) {
    ...
} BOOST_LOCAL_FUNCTION_NAME(f)

(4) The default parameter value cat(":", " ") is instead fine because it contains a comma , which is already wrapped by the parenthesis () of the function call cat(...).

Consider the following complete example (see also macro_commas.cpp):

void BOOST_LOCAL_FUNCTION(
    BOOST_IDENTITY_TYPE((const std::map<std::string, size_t>&)) m,
    BOOST_IDENTITY_TYPE((::sign_t)) sign,
    const size_t& factor,
            default (key_sizeof<std::string, size_t>::value),
    const std::string& separator, default cat(":", " ")
) {
    // Do something...
} BOOST_LOCAL_FUNCTION_NAME(f)

Local functions are function objects so it is possible to assign them to other functors like Boost.Function's boost::function in order to store the local function into a variable, pass it as a parameter to another function, or return it from the enclosing function.

For example (see also return_assign.cpp):

void call1(boost::function<int (int) > f) { BOOST_TEST(f(1) == 5); }
void call0(boost::function<int (void)> f) { BOOST_TEST(f() == 5); }

boost::function<int (int, int)> linear(const int& slope) {
    int BOOST_LOCAL_FUNCTION(const bind& slope,
            int x, default 1, int y, default 2) {
        return x + slope * y;
    } BOOST_LOCAL_FUNCTION_NAME(lin)

    boost::function<int (int, int)> f = lin; // Assign to local variable.
    BOOST_TEST(f(1, 2) == 5);

    call1(lin); // Pass to other functions.
    call0(lin);

    return lin; // Return.
}

void call(void) {
    boost::function<int (int, int)> f = linear(2);
    BOOST_TEST(f(1, 2) == 5);
}

[Warning] Warning

As with C++11 lambda functions, programmers are responsible to ensure that bound variables are valid in any scope where the local function object is called. Returning and calling a local function outside its declaration scope will lead to undefined behaviour if any of the bound variable is no longer valid in the scope where the local function is called (see the Examples section for more examples on the extra care needed when returning a local function as a closure). It is always safe instead to call a local function within its enclosing scope.

In addition, a local function can bind and call other local functions. Local functions should always be bound by constant reference const bind& to avoid unnecessary copies. For example, the following local function inc_sum binds the local function inc so inc_sum can call inc (see aslo transform.cpp):

int offset = 5;
std::vector<int> v;
std::vector<int> w;

for(int i = 1; i <= 2; ++i) v.push_back(i * 10);
BOOST_TEST(v[0] == 10); BOOST_TEST(v[1] == 20);
w.resize(v.size());

int BOOST_LOCAL_FUNCTION(const bind& offset, int i) {
    return ++i + offset;
} BOOST_LOCAL_FUNCTION_NAME(inc)

std::transform(v.begin(), v.end(), w.begin(), inc);
BOOST_TEST(w[0] == 16); BOOST_TEST(w[1] == 26);

int BOOST_LOCAL_FUNCTION(bind& inc, int i, int j) {
    return inc(i + j); // Call the other bound local function.
} BOOST_LOCAL_FUNCTION_NAME(inc_sum)

offset = 0;
std::transform(v.begin(), v.end(), w.begin(), v.begin(), inc_sum);
BOOST_TEST(v[0] == 27); BOOST_TEST(v[1] == 47);

It is possible to nest local functions into one another. For example (see also nesting.cpp):

int x = 0;

void BOOST_LOCAL_FUNCTION(bind& x) {
    void BOOST_LOCAL_FUNCTION(bind& x) { // Nested.
        x++;
    } BOOST_LOCAL_FUNCTION_NAME(g)

    x--;
    g(); // Nested local function call.
} BOOST_LOCAL_FUNCTION_NAME(f)

f();

This library never requires to explicitly specify the type of bound variables (e.g., this reduces maintenance because the local function declaration and definition do not have to change even if the bound variable types change as long as the semantics of the local function remain valid). From within local functions, programmers can access the type of a bound variable using the following macro:

BOOST_LOCAL_FUNCTION_TYPEOF(bound-variable-name)

The BOOST_LOCAL_FUNCTION_TYPEOF macro expands to a type expression that evaluates (at compile-time) to the fully qualified type of the bound variable with the specified name. This type expression is fully qualified in the sense that it will be constant if the variable is bound by constant const bind[&] and it will also be a reference if the variable is bound by reference [const] bind& (if needed, programmers can remove the const and & qualifiers using boost::remove_const and boost::remove_reference, see Boost.TypeTraits).

The deduced bound type can be used within the body to check concepts, declare local variables, etc. For example (see also typeof.cpp and addable.hpp):

int sum = 0, factor = 10;

void BOOST_LOCAL_FUNCTION(const bind factor, bind& sum, int num) {
    // Type-of for concept checking.
    BOOST_CONCEPT_ASSERT((Addable<boost::remove_reference<
            BOOST_LOCAL_FUNCTION_TYPEOF(sum)>::type>));
    // Type-of for declarations.
    boost::remove_reference<BOOST_LOCAL_FUNCTION_TYPEOF(
            factor)>::type mult = factor * num;
    sum += mult;
} BOOST_LOCAL_FUNCTION_NAME(add)

add(6);

Within templates, BOOST_LOCAL_FUNCTION_TYPEOF should not be prefixed by the typename keyword but eventual type manipulations need the typename prefix as usual (see also typeof_template.cpp and addable.hpp):

template<typename T>
T calculate(const T& factor) {
    T sum = 0;

    void BOOST_LOCAL_FUNCTION_TPL(const bind factor, bind& sum, T num) {
        // Local function `TYPEOF` does not need `typename`.
        BOOST_CONCEPT_ASSERT((Addable<typename boost::remove_reference<
                BOOST_LOCAL_FUNCTION_TYPEOF(sum)>::type>));
        sum += factor * num;
    } BOOST_LOCAL_FUNCTION_NAME_TPL(add)

    add(6);
    return sum;
}

In this context, it is best to use the BOOST_LOCAL_FUNCTION_TYPEOF macro instead of using Boost.Typeof to reduce the number of times that Boost.Typeof is invoked (either the library already internally used Boost.Typeof once, in which case using this macro will not use Boost.Typeof again, or the bound variable type is explicitly specified by programmers as shown be below, in which case using this macro will not use Boost.Typeof at all).

Furthermore, within the local function body it possible to access the result type using result_type, the type of the first parameter using arg1_type, the type of the second parameter using arg2_type, etc. [21]

While not required, it is possible to explicitly specify the type of bound variables so the library will not internally use Boost.Typeof to automatically deduce the types. When specified, the bound variable type must follow the bind "keyword" and it must be wrapped within round parenthesis ():

bind(variable-type) variable-name           // Bind by value with explicit type.
bind(variable-type)& variable-name          // Bind by reference with explicit type.
const bind(variable-type) variable-name     // Bind by constant value with explicit type.
const bind(variable-type)& variable-name    // Bind by constant reference with explicit type.
bind(class-type*) this_                     // Bind object `this` with explicit type.
const bind(class-type*) this_               // Bind object `this` by constant with explicit type.

Note that within the local function body it is always possible to abstract the access to the type of a bound variable using BOOST_LOCAL_FUNCTION_TYPEOF (even when the bound variable type is explicitly specified in the local function declaration).

The library also uses Boost.Typeof to determine the local function result type (because this type is specified outside the BOOST_LOCAL_FUNCTION macro). Thus it is also possible to specify the local function result type as one of the BOOST_LOCAL_FUNCTION macro parameters prefixing it by return so the library will not use Boost.Typeof to deduce the result type:

BOOST_LOCAL_FUNCTION_TYPE(return result-type, ...)

Note that the result type must be specified only once either before the macro (without the return prefix) or as one of the macro parameters (with the return prefix). As always, the result type can be void to declare a function that returns nothing (so return void is allowed when the result type is specified as one of the macro parameters).

The following example specifies all bound variables and result types (see also add_typed.cpp): [22]

struct adder {
    adder(void) : sum_(0) {}

    int sum(const std::vector<int>& nums, const int& factor = 10) {
        // Explicitly specify bound variable and return types (no type-of).
        BOOST_LOCAL_FUNCTION(const bind(const int&) factor,
                bind(adder*) this_, int num, return int) {
            return this_->sum_ += factor * num;
        } BOOST_LOCAL_FUNCTION_NAME(add)

        std::for_each(nums.begin(), nums.end(), add);
        return sum_;
    }

private:
    int sum_;
};

Unless necessary, it is recommended to not specify the bound variable and result types. Let the library deduce these types so the local function syntax will be more concise and the local function declaration will not have to change if a bound variable type changes (reducing maintenance).

[Note] Note

When all bound variable and result types are explicitly specified, the library implementation will not use Boost.Typeof.

Local functions can be declared inline to increase the chances that the compiler will be able to reduce the run-time of the local function call by inlining the generated assembly code. A local function is declared inline by prefixing its name with the keyword inline:

result-type BOOST_LOCAL_FUNCTION(parameters) {
    ... // Body.
} BOOST_LOCAL_FUNCTION_NAME(inline name) // Inlining.

When inlining a local function, note the following:

  • On C++03 compliant compilers, inline local functions always have a run-time comparable to their equivalent implementation that uses local functors (see the Alternatives section). However, inline local functions have the important limitation that they cannot be assigned to other functors (like boost::function) and they cannot be passed as template parameters.
  • On C++11 compilers, inline has no effect because this library will automatically generate code that uses C++11 specific features to inline the local function calls whenever possible even if the local function is not declared inline. Furthermore, non C++11 local functions can always be passes as template parameters even when they are declared inline. [23]
[Important] Important

It is recommended to not declare a local function inline unless it is strictly necessary for optimizing pure C++03 compliant code (because in all other cases this library will automatically take advantage of C++11 features to optimize the local function calls while always allowing to pass the local function as a template parameter).

For example, the following local function is declared inline (thus a for-loop needs to be used for portability instead of passing the local function as a template parameter to the std::for_each algorithm, see also add_inline.cpp):

int sum = 0, factor = 10;

void BOOST_LOCAL_FUNCTION(const bind factor, bind& sum, int num) {
    sum += factor * num;
} BOOST_LOCAL_FUNCTION_NAME(inline add) // Inlining.

std::vector<int> v(100);
std::fill(v.begin(), v.end(), 1);

for(size_t i = 0; i < v.size(); ++i) add(v[i]); // Cannot use for_each.

Local functions can be declared recursive so a local function can recursively call itself from its body (as usual with C++ functions). A local function is declared recursive by prefixing its name with the recursive "keyword" (thus recursive cannot be used as a local function name):

result-type BOOST_LOCAL_FUNCTION(parameters) {
    ... // Body.
} BOOST_LOCAL_FUNCTION_NAME(recursive name) // Recursive.

For example, the following local function is used to recursively calculate the factorials of all the numbers in the specified vector (see also factorial.cpp):

struct calculator {
    std::vector<int> results;

    void factorials(const std::vector<int>& nums) {
        int BOOST_LOCAL_FUNCTION(bind this_, int num,
                bool recursion, default false) {
            int result = 0;

            if(num <= 0) result = 1;
            else result = num * factorial(num - 1, true); // Recursive call.

            if(!recursion) this_->results.push_back(result);
            return result;
        } BOOST_LOCAL_FUNCTION_NAME(recursive factorial) // Recursive.

        std::for_each(nums.begin(), nums.end(), factorial);
    }
};

Compilers have not been observed to be able to inline recursive local function calls not even when the recursive local function is also declared inline:

... BOOST_LOCAL_FUNCTION_NAME(inline recursive factorial)

Recursive local functions should never be called outside their declaration scope. [24]

[Warning] Warning

If a local function is returned from the enclosing function and called in a different scope, the behaviour is undefined (and it will likely result in a run-time error).

This is not a limitation with respect to C++11 lambda functions because lambdas can never call themselves recursively (in other words, there is no recursive lambda function that can successfully be called outside its declaration scope because there is no recursive lambda function at all).

Because local functions are functors, it is possible to overload them using the boost::overloaded_function functor of Boost.Functional/OverloadedFunction from the boost/functional/overloaded_function.hpp header (see the Boost.Functional/OverloadedFunction documentation for details).

In the following example, the overloaded function object add can be called with signatures from either the local function add_s, or the local function add_d, or the local function add_d with its extra default parameter, or the function pointer add_i (see also overload.cpp):

int add_i(int x, int y) { return x + y; }

std::string s = "abc";
std::string BOOST_LOCAL_FUNCTION(
        const bind& s, const std::string& x) {
    return s + x;
} BOOST_LOCAL_FUNCTION_NAME(add_s)

double d = 1.23;
double BOOST_LOCAL_FUNCTION(const bind d, double x, double y, default 0) {
    return d + x + y;
} BOOST_LOCAL_FUNCTION_NAME(add_d)

boost::overloaded_function<
      std::string (const std::string&)
    , double (double)
    , double (double, double) // Overload giving default param.
    , int (int, int)
> add(add_s, add_d, add_d, add_i); // Overloaded function object.

BOOST_TEST(add("xyz") == "abcxyz"); // Call `add_s`.
BOOST_TEST(fabs(add(3.21) - 4.44) < 0.001); // Call `add_d` (no default).
BOOST_TEST(fabs(add(3.21, 40.0) - 44.44) < 0.001); // Call `add_d`.
BOOST_TEST(add(1, 2) == 3); // Call `add_i`.

It is possible to program exception specifications for local functions by specifying them after the BOOST_LOCAL_FUNCTION macro and before the body code block { ... }.

[Important] Important

Note that the exception specifications only apply to the body code specified by programmers and they do not apply to the rest of the code automatically generated by the macro expansions to implement local functions. For example, even if the body code is specified to throw no exception using throw () { ... }, the execution of the library code automatically generated by the macros could still throw (if there is no memory, etc).

For example (see also add_except.cpp):

double sum = 0.0;
int factor = 10;

void BOOST_LOCAL_FUNCTION(const bind factor, bind& sum,
        double num) throw() { // Throw nothing.
    sum += factor * num;
} BOOST_LOCAL_FUNCTION_NAME(add)

add(100);

Local function parameters support the storage classifiers as usual in C++03. The auto storage classifier is specified as: [25]

auto parameter-type parameter-name

The register storage classifier is specified as:

register parameter-type parameter-name

For example (see also add_classifiers.cpp):

int BOOST_LOCAL_FUNCTION(auto int x, register int y) { // Classifiers.
    return x + y;
} BOOST_LOCAL_FUNCTION_NAME(add)

In general, it is not possible to expand the BOOST_LOCAL_FUNCTION, BOOST_LOCAL_FUNCTION_TPL macros multiple times on the same line. [26]

Therefore, this library provides additional macros BOOST_LOCAL_FUNCTION_ID and BOOST_LOCAL_FUNCTION_TPL_ID which can be expanded multiple times on the same line as long as programmers specify unique identifiers as the macros' first parameters. The unique identifier can be any token (not just numeric) that can be successfully concatenated by the preprocessor (e.g., local_function_number_1_at_line_123). [27]

The BOOST_LOCAL_FUNCTION_ID and BOOST_LOCAL_FUNCTION_TPL_ID macros accept local function parameter declaration lists using the exact same syntax as BOOST_LOCAL_FUNCTION. For example (see also same_line.cpp):

#define LOCAL_INC_DEC(offset) \
    int BOOST_LOCAL_FUNCTION_ID(BOOST_PP_CAT(inc, __LINE__), /* unique ID */ \
            const bind offset, const int x) { \
        return x + offset; \
    } BOOST_LOCAL_FUNCTION_NAME(inc) \
    \
    int BOOST_LOCAL_FUNCTION_ID(BOOST_PP_CAT(dec, __LINE__), \
            const bind offset, const int x) { \
        return x - offset; \
    } BOOST_LOCAL_FUNCTION_NAME(dec)

#define LOCAL_INC_DEC_TPL(offset) \
    int BOOST_LOCAL_FUNCTION_TPL_ID(BOOST_PP_CAT(inc, __LINE__), \
            const bind offset, const int x) { \
        return x + offset; \
    } BOOST_LOCAL_FUNCTION_NAME(inc) \
    \
    int BOOST_LOCAL_FUNCTION_TPL_ID(BOOST_PP_CAT(dec, __LINE__), \
            const bind offset, const int x) { \
        return x - offset; \
    } BOOST_LOCAL_FUNCTION_NAME(dec)

template<typename T>
void f(int delta) {
    LOCAL_INC_DEC_TPL(delta) // Multiple local functions on same line.
    /** @todo back to BOOST_TEST(dec(inc(123)) == 123)) and doc that on vacpp compiler composition doesn't work */
    BOOST_TEST(inc(123) == 123 + delta);
    BOOST_TEST(dec(123) == 123 - delta);
}

int main(void) {
    int delta = 10;

    LOCAL_INC_DEC(delta) // Multiple local functions on same line.
    /** @todo back to BOOST_TEST(dec(inc(123)) == 123)) and doc that on vacpp compiler composition doesn't work */
    BOOST_TEST(inc(123) == 123 + delta);
    BOOST_TEST(dec(123) == 123 - delta);

    f(delta);
    return boost::report_errors();
}

As shown by the example above, the BOOST_LOCAL_FUNCTION_ID and BOOST_LOCAL_FUNCTION_TPL_ID macros are especially useful when it is necessary to invoke them multiple times within a user-defined macro (because the preprocessor expands all nested macros on the same line).

The following table summarizes all C++ function features indicating those features that are not supported by this library for local functions.

C++ Function Feature

Local Function Support

Comment

export

No.

This is not supported because local functions cannot be templates (plus most C++ compilers do not implement export at all).

template<template-parameter-list>

No.

This is not supported because local functions are implemented using local classes and C++03 local classes cannot be templates.

explicit

No.

This is not supported because local functions are not constructors.

inline

Yes.

Local functions can be specified inline to improve the chances that C++03 compilers can optimize the local function call run-time (but inline local functions cannot be passed as template parameters on C++03 compilers, see the Advanced Topics section).

extern

No.

This is not supported because local functions are always defined locally within the enclosing scope and together with their declarations.

static

No.

This is not supported because local functions are not member functions.

virtual

No.

This is not supported because local functions are not member functions. [a]

result-type

Yes.

This is supported (see the Tutorial section).

function-name

Yes.

Local functions are named and they can call themselves recursively but they cannot be operators (see the Tutorial and Advanced Topics sections).

parameter-list

Yes.

This is supported and it also supports the auto and register storage classifiers, default parameters, and binding of variables in scope (see the Tutorial and Advanced Topics sections).

Trailing const qualifier

No.

This is not supported because local functions are not member functions.

Trailing volatile qualifier

No.

This is not supported because local functions are not member functions.

[a] Rationale. It would be possible to make a local function class inherit from another local function class. However, this "inheritance" feature is not implemented because it seemed of no use given that local functions can be bound to one another thus they can simply call each other directly without recurring to dynamic binding or base function calls.

Operators

Local functions cannot be operators. Naming a local function operator... will generate a compile-time error. [28]

For example, the following code does not compile (see also operator_error.cpp):

bool BOOST_LOCAL_FUNCTION(const point& p, const point& q) {
    return p.x == q.x && p.y == q.y;
} BOOST_LOCAL_FUNCTION_NAME(operator==) // Error: Cannot use `operator...`.

Goto

It is possible to jump with a goto within the local function body. For example, the following compiles (see also goto.cpp):

int error(int x, int y) {
    int BOOST_LOCAL_FUNCTION(int z) {
        if(z > 0) goto success; // OK: Can jump within local function.
        return -1;
    success:
        return 0;
    } BOOST_LOCAL_FUNCTION_NAME(validate)

    return validate(x + y);
}

However, it is not possible to jump with a goto from within the local function body to to a label defined in the enclosing scope. For example, the following does not compile (see also goto_error.cpp):

int error(int x, int y) {
    int BOOST_LOCAL_FUNCTION(int z) {
        if(z <= 0) goto failure;    // Error: Cannot jump to enclosing scope.
        else goto success;          // OK: Can jump within local function.
    success:
        return 0;
    } BOOST_LOCAL_FUNCTION_NAME(validate)

    return validate(x + y);
failure:
    return -1;
}



[17] Rationale. The assignment symbol = cannot be used to specify default parameter values because default values are not part of the parameter type so they cannot be handled using template meta-programming. Default parameter values need to be separated from the rest of the parameter declaration using the preprocessor. Specifically, this library needs to use preprocessor meta-programming to remove default values when constructing the local function type and also to count the number of default values to provide the correct set of call operators for the local functor. Therefore, the symbol = cannot be used because it cannot be handled by preprocessor meta-programming (non-alphanumeric symbols cannot be detected by preprocessor meta-programming because they cannot be concatenated by the preprocessor).

[18] The authors do not personally find the use of the WITH_DEFAULT macro more readable and they prefer to use the default keyword directly. Furthermore, WITH_DEFAULT needs to be defined differently for compilers without variadic macros #define WITH_DEFAULT (default) so it can only be defined by programmers based on the syntax they decide to use (see the No Variadic Macros section).

[19] Rationale. This limitation is because this library uses preprocessor token concatenation ## to inspect the macro parameters (to distinguish between function parameters, bound variables, etc) and the C++ preprocessor does not allow to concatenate non-alphanumeric tokens.

[20] The preprocessor always interprets unwrapped commas as separating macro parameters. Thus in this case the comma will indicate to the preprocessor that the first macro parameter is const std::map<std::tring, the second macro parameter is size_t>& m, etc instead of passing const std::map<std::string, size_t>& m as a single macro parameter.

[21] Rationale. The type names result_type and argN_type follow the Boost.TypeTraits naming conventions for function traits.

[22] In the examples of this documentation, bound variables, function parameters, and the result type are specified in this order because this is the order used by C++11 lambda functions. However, the library accepts bound variables, function parameters, and the result type in any order.

[23] Rationale. This library uses an indirect function call via a function pointer in order to pass the local function as a template parameter (see the Implementation section). No compiler has yet been observed to be able to inline function calls when they use such indirect function pointer calls. Therefore, inline local functions do not use such indirect function pointer call (so they are more likely to be optimized) but because of that they cannot be passed as template parameters. The indirect function pointer call is needed on C++03 but it is not needed on C++11 (see [N2657] and Boost.Config's BOOST_NO_LOCAL_CLASS_TEMPLATE_PARAMETERS) thus this library automatically generates local function calls that can be inline on C++11 compilers (even when the local function is not declared inline).

[24] Rationale. This limitation comes from the fact that the global functor used to pass the local function as a template parameter (and eventually returned outside the declarations scope) does not know the local function name so the local function name used for recursive call cannot be set in the global functor. This limitation together with preventing the possibility for inlining are the reasons why local functions are not recursive unless programmers explicitly declare them recursive.

[25] The auto storage classifier is part of the C++03 standard and therefore supported by this library. However, the meaning and usage of the auto keyword changed in C++11. Therefore, use the auto storage classifier with the usual care in order to avoid writing C++03 code that might not work on C++11.

[26] Rationale. The BOOST_LOCAL_FUNCTION and BOOST_LOCAL_FUNCTION_TPL macros internally use __LINE__ to generate unique identifiers. Therefore, if these macros are expanded more than on time on the same line, the generated identifiers will no longer be unique and the code will not compile. (This restriction does not apply to MSVC and other compilers that provide the non-standard __COUNTER__ macro.) Note that the BOOST_LOCAL_FUNCTION_NAME macro can always be expanded multiple times on the same line because the unique local function name (and not __LINE__) is used by this macro to generate unique identifiers (so there is no need for a BOOST_LOCAL_FUNCTION_NAME_ID macro).

[27] Because there are restrictions on the set of tokens that the preprocessor can concatenate and because not all compilers correctly implement these restrictions, it is in general recommended to specify unique identifiers as a combination of alphanumeric tokens.

[28] Rationale. This is the because a local function name must be a valid local variable name (the local variable used to hold the local functor) and operators cannot be used as local variable names.


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