Visual C ++编译器允许dependent-name作为没有“typename”的类型？ [英] Visual C++ Compiler allows dependent-name as a type without "typename"?

问题描述

今天我的一个朋友告诉我，下面的代码在他的Visual Studio 2008中编译得很好：

Today one of my friends told me that the following code compiles well on his Visual Studio 2008:

#include <vector>
struct A
{
  static int const const_iterator = 100;
};
int i;
template <typename T>
void PrintAll(const T & obj)
{
  T::const_iterator *i;
}
int main()
{
  std::vector<int> v;
  A a;
  PrintAll(a);
  PrintAll(v);
  return 0;
}

我通常使用g ++，它总是拒绝通过第二个PrintAll呼叫。我知道，对于这个问题，g ++是做标准的方式翻译一个模板。

I usually use g++, and it always refuse to pass the second PrintAll() call. As I know, for this problem, g++ is doing the standard way translating a template.

那么，我的知识是错误的，还是VS2008的扩展？ p>

So, is my knowledge wrong, or is it a extension of VS2008?

推荐答案

这不是一个扩展。

VC ++从来没有实现两个阶段适当地解释：

VC++ never implemented the two phases interpretation properly:

1. 在定义点，解析模板并确定所有非依赖名称


2. 在实例化时，检查模板是否生成有效的代码

VC ++从未实现第一个阶段...不方便，因为它意味着它不仅接受不符合的代码，而且在某些情况下它产生完全不同的代码。

VC++ never implemented the first phase... it's inconvenient since it means not only that it accepts code that is non-compliant but also that it produces an altogether different code in some situations.

void foo(int) { std::cout << "int" << std::endl; }

template <class T> void tfoo() { foo(2.0); }

void foo(double) { std::cout << "double" << std::endl; }

int main(int argc, char* argv[])
{
  tfoo<Dummy>();
}

使用此代码：

• 兼容的编译器将打印int，因为它是模板定义时可用的唯一定义，并且 foo 不依赖于 T 。


• VC ++将打印double，因为它从不打扰第1阶段

• compliant compilers will print "int", because it was the only definition available at the point of definition of the template and the resolution of foo does not depend on T.
• VC++ will print "double", because it never bothered with phase 1

差异看起来可能显得很愚蠢，但如果你考虑一个大型程序中包含的数量，将在您的模板代码...和BAM：/

It might seem stupid as far as differences go, but if you think about the number of includes you have in a large program, there is a risk that someone will introduce an overload after your template code... and BAM :/

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