模板不同的返回类型

template different return types

本文关键字：返回类型      更新时间：2023-10-16

在下面的代码中，我必须将boolean_cast转换为TDestination以避免编译器警告（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;
}

正如@n.m在一条注释中所说，编译器不会停止编译if的假分支，因此当目标类型为bool和目标类型为VARIANT_BOOL时，这两个分支都必须是有效代码。

您可以通过将调度到部分专用于目标类型的帮助模板来避免该问题，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_;
}
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