使用模板参数的value_type [英] Using a templated parameter's value_type

问题描述

如何使用std容器的value_type？

我试着使用它：

How is one supposed to use a std container's value_type?
I tried to use it like so:

#include <vector>

using namespace std;

template <typename T>
class TSContainer {
private:
        T container;
public:
        void push(T::value_type& item)
        {
                container.push_back(item);
        }
        T::value_type pop()
        {
                T::value_type item = container.pop_front();
                return item;
        }
};
int main()
{
        int i = 1;
        TSContainer<vector<int> > tsc;
        tsc.push(i);
        int v = tsc.pop();
}

但这会导致：

prog.cpp:10: error: ‘T::value_type’ is not a type
prog.cpp:14: error: type ‘T’ is not derived from type ‘TSContainer<T>’
prog.cpp:14: error: expected ‘;’ before ‘pop’
prog.cpp:19: error: expected `;' before ‘}’ token
prog.cpp: In function ‘int main()’:
prog.cpp:25: error: ‘class TSContainer<std::vector<int, std::allocator<int> > >’ has no member named ‘pop’
prog.cpp:25: warning: unused variable ‘v’

b $ b

我认为这是什么:: value_type是为了？

I thought this was what ::value_type was for?

推荐答案

c> typename ：

You have to use typename:

typename T::value_type pop()

等等。

原因是编译器无法知道T :: value_type是一个成员变量的类型（没有人阻止你定义一个类型 struct X {int value_type;}; 并将其传递给模板）。但是没有该函数，代码不能被解析（因为构造的含义取决于某些标识符是否指定类型或变量，例如 T * p 可能是乘法或指针声明）。因此，规则是，可以是类型或变量，并且没有通过用 typename 前缀明确标记为类型的所有内容都被认为是一个变量。

The reason is that the compiler cannot know whether T::value_type is a type of a member variable (nobody hinders you from defining a type struct X { int value_type; }; and pass that to the template). However without that function, the code could not be parsed (because the meaning of constructs changes depending on whether some identifier designates a type or a variable, e.g.T * p may be a multiplication or a pointer declaration). Therefore the rule is that everything which might be either type or variable and is not explicitly marked as type by prefixing it with typename is considered a variable.

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