无法推导模板参数 [英] Template parameter cannot be deduced
问题描述
我不明白为什么在这种情况下不能推断出T:
I don't understand why T cannot be deduced in this scenario:
template<class T>
class MyType
{
T * data;
};
class MyOtherType
{
};
template<typename T>
struct MyType_OutArg
{
typedef MyType<T> & type;
};
template<typename T>
void
DoSomething(typename MyType_OutArg<T>::type obj)
{
}
void func(MyType_OutArg<MyOtherType>::type obj)
{
DoSomething(obj);
}
来自GCC 4.7.1,带有-std = c ++ 14
From GCC 4.7.1 with -std=c++14
<source>: In function 'void func(MyType_OutArg<MyOtherType>::type)':
26 : <source>:26:20: error: no matching function for call to 'DoSomething(MyType<MyOtherType>&)'
DoSomething(obj);
^
26 : <source>:26:20: note: candidate is:
19 : <source>:19:1: note: template<class T> void DoSomething(typename MyType_OutArg<T>::type)
DoSomething(typename MyType_OutArg<T>::type obj)
^
19 : <source>:19:1: note: template argument deduction/substitution failed:
26 : <source>:26:20: note: couldn't deduce template parameter 'T'
DoSomething(obj);
^
Compiler returned: 1
当然,以下工作:
DoSomething<MyOtherType>(obj);
但我不确定为什么有必要。
but i'm unsure why it's necessary. Shouldn't the compiler have enough information?
推荐答案
这是因为您的情况是非推论上下文> em>。
This is because your case is a Non-deduced contexts.
引用自 http ://en.cppreference.com/w/cpp/language/template_argument_deduction :
非推论上下文
Non-deduced contexts
在以下情况下,用于构成P的类型,模板和非类型值不参与模板自变量的推导,而是使用在其他地方推导或明确指定。如果仅在非推导上下文中使用模板参数并且未明确指定模板参数,则模板参数推导将失败。
In the following cases, the types, templates, and non-type values that are used to compose P do not participate in template argument deduction, but instead use the template arguments that were either deduced elsewhere or explicitly specified. If a template parameter is used only in non-deduced contexts and is not explicitly specified, template argument deduction fails.
1)嵌套名称说明符(使用限定ID指定的类型的作用域解析运算符::)的左侧
1) The nested-name-specifier (everything to the left of the scope resolution operator ::) of a type that was specified using a qualified-id
在您的情况下, typename MyType_OutArg< T> :: type
将不参与类型推导,并且在其他地方也不知道 T
,因此此模板功能将被忽略。
In your case, typename MyType_OutArg<T>::type
will not participate in type deduction, and T
is not known from elsewhere, thus this template function is ignored.
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