[boost] [c++11] compile_if utility

4 Jul 2012

      Hello all,

I was thinking how to program assertion requirements in C++11. I need
to program something like this

    if "has_equal_to<T>" then "assert(back() == value)"
    else do nothing, don't even compile "assert(back() == value)"

Using C++11 lambdas, I was able to program a compile_if utility to do
the following:

    compile_if< boost::has_equal_to<T> >([&]() {
        assert(this->back() == value); // compiled and executed iff
has_equal_to<T>
    });

Any interest in adding compile_if to Boost.Utility? It could be
renamed exec_if if compile_if is too "strong".

I didn't play with compile_if much so there might cases where it
doesn't work... but I wanted to share it with the ML anyway:

#include <boost/type_traits.hpp>
#include <cassert>
#include <vector>
#include <iostream>

template< bool Boolean >
struct compile_if_c
{
    template< typename UnaryFunc >
    compile_if_c ( UnaryFunc f )
    {
        std::cout << "compiling" << std::endl;
        f();
    }
};

template< >
struct compile_if_c <false>
{
    compile_if_c ( ... )
    {
        std::cout << "not compiling" << std::endl;
    }
};

template< class BooleanMetafunc, class UnaryFunc >
void compile_if ( UnaryFunc f )
{
    compile_if_c<BooleanMetafunc::value> compile(f);
}

template< typename T >
class vector
{
    public: typedef typename std::vector<T>::const_reference const_reference;

    public: void push_back ( T const& value )
    {
        vector_.push_back(value);
        compile_if< boost::has_equal_to<T> >([&]() {
            assert(this->back() == value);
        });
    }

    public: const_reference back ( void ) const
    {
        return vector_.back();
    }

    private: std::vector<T> vector_;
};

struct x {}; // no operator==

int main ( void )
{
    vector<x>().push_back(x()); // no == so skip assertion back() == value
    vector<int>().push_back(123); // has == so assert back() == value
    return 0;
}

Thanks.
--Lorenzo

P.S. Eh, if C++11 lambda had constant captures [const&](){
assert(this->back() == value; } I could have even made the assertion
constant-correct...

[boost] [c++11] compile_if utility

Lorenzo Caminiti