Code C++ Functions to Prevent Clients Ignoring Return Values

Ignoring Function Return Values

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;
    }
};

This template is an example of Catching defects with Patterns.

Reference

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