模板:模板函数不能很好地与类的模板成员函数配

This is a minimal test case of some code that I actually have. It fails when it tries to evaluate a.getResult<B>():

test.cpp: In function 'void printStuff(const A&)':
test.cpp:6: error: expected primary-expression before '>' token
test.cpp:6: error: expected primary-expression before ')' token

代码是:

#include <iostream>

template< class A, class B>
void printStuff( const A& a)
{
    size_t value = a.getResult<B>();
    std::cout << value << std::endl;
}

struct Firstclass {
    template< class X >
    size_t getResult() const {
        X someInstance;
        return sizeof(someInstance);
    }
};

int main(int, char**) {
    Firstclass foo;

    printStuff<Firstclass, short int>(foo);
    printStuff<Firstclass, double>(foo);

    std::cout << foo.getResult< double >() << std::endl;

    return 0;
}

如果我注释掉 printStuff 函数及其调用位置，foo.getResult<double >() 调用编译良好，并符合预期.

If I comment out the printStuff function and where it's called, the foo.getResult< double >() call compiles fine and does what is expected.

知道发生了什么吗?一段时间以来，我一直在使用广泛的模板化代码，但从未遇到过这样的事情.

Any idea what's going on? I've been working with extensively templated code for a while and have never encountered anything like this.

推荐答案

当您引用属于依赖类型成员的模板时，您必须在它前面加上关键字 template.这就是对 printStuff 内的 getResult 的调用应该是什么样子

When you refer to a template that is a member of dependent type, you have to prepend it with a keyword template. This is how the call to getResult inside printStuff should look

size_t value = a.template getResult<B>();

这类似于在引用依赖类型中的嵌套类型名时使用关键字 typename.出于某种原因，关于typename 嵌套类型的部分是众所周知的，但是对于template 嵌套模板的类似要求却相对未知.

This is similar to using the keyword typename when referring to nested typenames in a dependent type. For some reason, the bit about typename with nested types is rather well-known, but the similar requirement for template with nested templates is relatively unknown.

请注意，一般的语法结构有点不同.typename 总是放在类型的全名前面，而 ​​template 插入在中间.

Note that the general syntax structure is a bit different though. The typename is always put in front of the full name of the type, while template is inserted in the middle.

同样，这仅在您访问依赖类型的模板成员时才需要，在上面的示例中为 printStuff 中的 A代码>.当您在 main 中调用 foo.getResult<> 时，foo 的类型是不相关的，因此不需要包含 模板关键字.

Again, this is only necessary when you are accessing a template member of a dependent type, which in the above example would be A in printStuff. When you call foo.getResult<> in main the type of foo is not dependent, so there's no need to include the template keyword.

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