...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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
, defaultparameter-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 | |
---|---|
Also macro parameters with commas wrapped by angular parenthesis |
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 | |
---|---|
Often, there might be better ways to overcome this limitation that lead
to code which is more readable than the one using the |
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 | |
---|---|
Often, there might be better ways to overcome this limitation that lead
to code which is more readable than the one using the |
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 | |
---|---|
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 | |
---|---|
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:
boost::function
)
and they cannot be passed as template parameters.
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 | |
---|---|
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 | |
---|---|
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((4.44 - add(3.21)) <= 0.001); // Call `add_d` (no default). BOOST_TEST((44.44 - add(3.21, 40.0)) <= 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 | |
---|---|
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 |
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
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_ID_TPL
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_ID_TPL
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) \ T BOOST_LOCAL_FUNCTION_ID_TPL(BOOST_PP_CAT(inc, __LINE__), \ const bind offset, const T x) { \ return x + offset; \ } BOOST_LOCAL_FUNCTION_NAME_TPL(inc) \ \ T BOOST_LOCAL_FUNCTION_ID_TPL(BOOST_PP_CAT(dec, __LINE__), \ const bind offset, const T x) { \ return x - offset; \ } BOOST_LOCAL_FUNCTION_NAME_TPL(dec) template<typename T> void f(T& delta) { LOCAL_INC_DEC_TPL(delta) // Multiple local functions on same line. BOOST_TEST(dec(inc(123)) == 123); } int main(void) { int delta = 10; LOCAL_INC_DEC(delta) // Multiple local functions on same line. BOOST_TEST(dec(inc(123)) == 123); f(delta); return boost::report_errors(); }
As shown by the example above, the BOOST_LOCAL_FUNCTION_ID
and BOOST_LOCAL_FUNCTION_ID_TPL
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 |
---|---|---|
|
No. |
This is not supported because local functions cannot be templates
(plus most C++ compilers do not implement |
|
No. |
This is not supported because local functions are implemented using local classes and C++03 local classes cannot be templates. |
|
No. |
This is not supported because local functions are not constructors. |
|
Yes. |
Local functions can be specified |
|
No. |
This is not supported because local functions are always defined locally within the enclosing scope and together with their declarations. |
|
No. |
This is not supported because local functions are not member functions. |
|
No. |
This is not supported because local functions are not member functions. [a] |
|
Yes. |
This is supported (see the Tutorial section). |
|
Yes. |
Local functions are named and they can call themselves recursively but they cannot be operators (see the Tutorial and Advanced Topics sections). |
|
Yes. |
This is supported and it also supports the |
Trailing |
No. |
This is not supported because local functions are not member functions. |
Trailing |
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. |
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...`.
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 arg
N
_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_CXX11_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.