C++ template to force use of function return value.

ignore.hpp

// If it is important that a return value not be ignored by the caller of a function,
// the template dont_ignore may be used as follows:
// BEFORE:
// int add( int x, int y )
// {
//     return x + y;
// }
// AFTER:
// dont_ignore<int> add( int x, int y )
// {
//     return x + y;
// }
//
// When the caller of the function does not use the return value,
// an exception is thrown. Definition of dont_ignore:

template<class T>
struct dont_ignore
{
    const T     v;
    bool        used;

    dont_ignore( const T& v )
        :  v( v ), used( false )
    {}

    ~dont_ignore()
    {
        if ( !used )
            throw std::runtime_error( "return value not used" );
    }

    operator T()
    {
        used = true;
        return v;
    }
};