模板不同的返回类型

Question

在下面的代码中，我必须将boolean_cast强制转换为TDestination，以避免出现编译器警告（从“ VARIANT_BOOL”截断为“ bool”）。 这是编译器问题还是C ++问题？

template<typename TDestination, typename TSource>
TDestination boolean_cast(TSource source)
{
    TDestination destination;

    static_assert(std::is_same<TDestination, bool>::value || std::is_same<TDestination, VARIANT_BOOL>::value, "destination must be bool or VARIANT_BOOL");

    //convert to bool
    if (std::is_same<TDestination, bool>::value)
    {
        if (source)
            destination = true;
        else
            destination = false;
    }
    //convert to VARIANT_BOOL
    else
    {
        if (source)
            destination = (TDestination)VARIANT_TRUE;
        else
            destination = (TDestination)VARIANT_FALSE;
    }

    return destination;
}

Answer 1

就像@nm在评论中说的那样，当目标类型为bool且目标类型为VARIANT_BOOL时，编译器不会停止编译if的false分支，因此两个分支都必须是有效代码。

您可以通过分派到专门针对目标类型的帮助程序模板来避免该问题

template<typename TDestination, typename TSource>
struct boolean_cast_impl;  // undefined

template<typename TSource>
struct boolean_cast_impl<bool, TSource>
{
    static bool cast(TSource source)
    {
        return source ? true : false;
    }
};

template<typename TSource>
struct boolean_cast_impl<VARIANT_BOOL, TSource>
{
    static VARIANT_BOOL cast(TSource source)
    {
       return source ? VARIANT_TRUE : VARIANT_FALSE;
    }
};

template<typename TDestination, typename TSource>
TDestination boolean_cast(TSource source)
{
    static_assert(std::is_same<TDestination, bool>::value || std::is_same<TDestination, VARIANT_BOOL>::value, "destination must be bool or VARIANT_BOOL");

    return boolean_cast_impl<TDestination, TSource>::cast(source);
}

甚至只是在专业化中定义正确类型的常量：

template<typename TDestination, typename TSource>
struct boolean_cast_values;  // undefined

template<typename TSource>
struct boolean_cast_values<bool, TSource>
{
    static const bool true_ = true;
    static const bool false_ = false;
};

template<typename TSource>
struct boolean_cast_values<VARIANT_BOOL, TSource>
{
    static const VARIANT_BOOL true_ = VARIANT_TRUE;
    static const VARIANT_BOOL false_ = VARIANT_FALSE;
};

template<typename TDestination, typename TSource>
TDestination boolean_cast(TSource source)
{
    static_assert(std::is_same<TDestination, bool>::value || std::is_same<TDestination, VARIANT_BOOL>::value, "destination must be bool or VARIANT_BOOL");

    typedef boolean_cast_values<TDestination, TSource> values;
    return source ? values::true_ : values::false_;
}