针对模板的有限和无限定名称查找的不同行为

问题描述

此代码应如何运行？如果在 call_read（）函数中使用限定名称，则调用忽略我的重载的泛型函数;如果我使用非限定名称，它会先调用重载，然后调用通用版本。有什么不同？这是GCC中的错误吗？

How should this code behave? It calls generic function ignoring my overload if I use qualified name in call_read() function; and it calls overload first and then generic version if I use unqualified name. What's the difference? Is it a bug in GCC?

#include <iostream>

struct info1 {};
struct info2 {};

template<class T> void read(T& x)
{
   std::cout << "generic" << std::endl;
}

template<class T> void call_read(T& x)
{
   ::read(x); // if I replace ::read(x) with read(x) the overload is called
}

void read(info1& x)
{
   std::cout << "overload" << std::endl;
}

int main()
{
   info1 x;
   info2 y;
   call_read(x);
   call_read(y);
}

我还注意到它对基本类型的工作不同。
查看下面的代码

I also noticed that it works different for fundamental types.See the code bellow

#include <iostream>

typedef struct info1 {};
typedef struct info2 {};
typedef int info3;
typedef double info4;

template<class T> void read(T x)
{
    std::cout << "generic" << std::endl;
}

template<class T> void call_read(T x)
{
    read(x);
}

void read(info1 x)
{
    std::cout << "overload" << std::endl;
}
void read(info3 x)
{
    std::cout << "overload" << std::endl;
}

int main()
{
    call_read(info1());
    call_read(info2());
    call_read(info3());
    call_read(info4());
}

它应该调用重载函数两次，但不是。
查看结果

It is supposed to call overloaded function twice, but it's not.See the result herehttp://codepad.org/iFOOFD52

推荐答案

您观察到的是两相名称查找和 / em>。

What you're observing is a superposition of two-phase name lookup and argument dependent lookup.

让我们看看标准说的是什么（C ++ 03）。 [temp.dep]：

Let's see what the standard says (C++03). [temp.dep]:

postfix-expression ( expression-listopt )

其中postfix-expression是标识符，当且仅当表达式列表中的任何表达式是类型依赖表达式（14.6.2.2）时，标识符表示依赖名称。

where the postfix-expression is an identifier, the identifier denotes a dependent name if and only if any of the expressions in the expression-list is a type-dependent expression (14.6.2.2).

这意味着在读和 :: read ， read 是一个依赖名称，因为 x 是类型相关的。这意味着它在实例化的时候解决。让我们看看这个[temp.dep.candidate]的规则是什么：

That means that in both read and ::read, read is a dependent name because x is type-dependent. That means that it's resolved at the point of instantiation. Let's see what are the rules for this [temp.dep.candidate]:

- 对于使用非限定名称查找（3.4.1）的查找部分，仅找到具有模板定义上下文的外部链接的函数声明。

— For the part of the lookup using unqualified name lookup (3.4.1), only function declarations with external linkage from the template definition context are found.

因此对于 :: read 情况，只考虑在模板定义之前声明的函数。但是：

Therefore for the ::read case only functions declared before the template definition are considered. But:

用于不合格的 read 两个函数，它们在模板定义和模板实例化时可见。

for the unqualified read both functions are considered, those visible at template definition and template instantiation.

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