Mixing Approaches

For purposes of exposition, we've divided the discussion of how to eliminate runtime penalties by the different approaches available. A realistic program could likely include all techniques mentioned above. Consider the following:

//  Copyright (c) 2018 Robert Ramey
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#include <stdexcept>
#include <iostream>
#include <sstream>

#include <boost/safe_numerics/safe_integer.hpp>
#include <boost/safe_numerics/safe_integer_range.hpp>
#include <boost/safe_numerics/native.hpp>
#include <boost/safe_numerics/exception.hpp>

#include "safe_format.hpp" // prints out range and value of any type

using namespace boost::safe_numerics;

using safe_t = safe_signed_range<
    -24,
    82,
    native,
    loose_exception_policy
>;

// define variables used for input
using input_safe_t = safe_signed_range<
    -24,
    82,
    native, // we don't need automatic in this case
    loose_exception_policy // assignment of out of range value should throw
>;

// function arguments can never be outside of limits
auto f(const safe_t & x, const safe_t & y){
    auto z = x + y;  // we know that this cannot fail
    std::cout << "z = " << safe_format(z) << std::endl;
    std::cout << "(x + y) = " << safe_format(x + y) << std::endl;
    std::cout << "(x - y) = " << safe_format(x - y) << std::endl;
    return z;
}

bool test(const char * test_string){
    std::istringstream sin(test_string);
    input_safe_t x, y;
    try{
        std::cout << "type in values in format x y:" << test_string << std::endl;
        sin >> x >> y; // read varibles, maybe throw exception
    }
    catch(const std::exception & e){
        // none of the above should trap. Mark failure if they do
        std::cout << e.what() << '\n' << "input failure" << std::endl;
        return false;
    }
    std::cout << "x" << safe_format(x) << std::endl;
    std::cout << "y" << safe_format(y) << std::endl;
    std::cout << safe_format(f(x, y)) << std::endl;
    std::cout << "input success" << std::endl;
    return true;
}

int main(){
    std::cout << "example 84:\n";
    bool result =
        !  test("123 125")
        && test("0 0")
        && test("-24 82")
    ;
    std::cout << (result ? "Success!" : "Failure") << std::endl;
    return result ? EXIT_SUCCESS : EXIT_FAILURE;
}

As before, we define a type safe_t to reflect our view of legal values for this program. This uses the automatic type promotion policy as well as the loose_trap_policy exception policy to enforce elimination of runtime penalties.
The function f accepts only arguments of type safe_t so there is no need to check the input values. This performs the functionality of programming by contract with no runtime cost.
In addition, we define input_safe_t to be used when reading variables from the program console. Clearly, these can only be checked at runtime so they use the throw_exception policy. When variables are read from the console they are checked for legal values. We need no ad hoc code to do this, as these types are guaranteed to contain legal values and will throw an exception when this guarantee is violated. In other words, we automatically get checking of input variables with no additional programming.
On calling of the function f, arguments of type input_safe_t are converted to values of type safe_t . In this particular example, it can be determined at compile time that construction of an instance of a safe_t from an input_safe_t can never fail. Hence, no try/catch block is necessary. The usage of the loose_trap_policy policy for safe_t types guarantees this to be true at compile time.

Here is the output from the program when values 12 and 32 are input from the console:

example 84:
type in values in format x y:33 45
x<signed char>[-24,82] = 33
y<signed char>[-24,82] = 45
z = <short>[-48,164] = 78
(x + y) = <short>[-48,164] = 78
(x - y) = <signed char>[-106,106] = -12
<short>[-48,164] = 78

Boost C++ Libraries

Mixing Approaches