SFINAE 尝试使用 bool 给出编译器错误:“模板参数‘T::value’涉及模板参数";
我尝试使用 bool
(不同于流行的 void_
技巧):
模板结构解析{静态常量布尔值 = 假;};模板struct Resolve{静态常量布尔值 = 真;};
目标是专门化,其中定义了 static const bool my_value = true;
的类.如果它们已定义 false
或未定义,则不要专门化它.即
struct B1 {//专门针对这种情况进行解析static const bool my_value = true;};struct B2 {//不特化static const bool my_value = false;};结构 B3 {};//不专业
当在 B1
上应用上述技巧时,它给出了编译错误:
解析::value;
<块引用>
错误:模板参数‘T::my_value’涉及模板参数
我知道这可以通过其他方式实现.但是,我很想知道,为什么它会在此处给出编译器错误,并且可以在此代码本身中解决吗?
解决方案实际上你在做什么是第 §14.5.4/9 节禁止的,它说,
<块引用>部分特化的非类型参数表达式不应涉及部分特化的模板参数,除非参数表达式是一个简单的标识符.
技巧也可以使用 type 作为第二个模板参数,封装 非类型 值,如下所述:
template结构布尔类型{};template>结构解析{静态常量布尔值 = 假;};模板struct Resolve>{静态常量布尔值 = 真;};
现在它编译罚款.
I tried to implement an SFINAE using bool
(unlike popular void_
trick):
template<typename T, bool = true>
struct Resolve
{
static const bool value = false;
};
template<typename T>
struct Resolve<T, T::my_value>
{
static const bool value = true;
};
The goal is to specialize, the classes which have static const bool my_value = true;
defined inside it. If they are defined false
or not defined then don't specialize it. i.e.
struct B1 { // specialize Resolve for this case
static const bool my_value = true;
};
struct B2 { // don't specialize
static const bool my_value = false;
};
struct B3 {}; // don't specialize
When applying the above trick on B1
it gives the compilation error:
Resolve<B1>::value;
error: template argument ‘T::my_value’ involves template parameter(s)
I am aware that this can be achieved with alternate ways. However, I am interested in knowing, why it gives compiler error here and can it be solved in this code itself ?
解决方案Actually what you're doing is forbidden by section §14.5.4/9 which says,
A partially specialized non-type argument expression shall not involve a template parameter of the partial specialization except when the argument expression is a simple identifier.
The trick could be using a type for second template parameter as well, encapsulating the non-type value, as described below:
template<bool b> struct booltype {};
template<typename T, typename B = booltype<true> >
struct Resolve
{
static const bool value = false;
};
template<typename T>
struct Resolve<T, booltype<T::my_value> >
{
static const bool value = true;
};
Now it compile fines.
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