Question

在下面的代码中，我必须使用boolean_cast转换为TDestination以避免编译器警告（从'VARIANT_BOOL'截断到'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

as @ n.m。在评论中说，编译器不会停止编译if的假分支，因此当目标类型为bool且目标类型为VARIANT_BOOL时，两个分支必须是有效代码

您可以通过调度部分专门针对目标类型的帮助程序模板来避免此问题，例如。

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_;
}

模板不同的返回类型

1 个答案: