C++语法歧义

发布于 2024-10-31 06:59:56 字数 331 浏览 0 评论 0原文

考虑一下：

void f(std::pair<bool,bool> terms = std::pair<bool,bool>(1,1)) {}

gcc 4.4 没问题，gcc 4.3 抱怨 错误：在 '>' 之前预期有 ',' 或 '...'令牌。解决办法是：

void f(std::pair<bool,bool> terms = (std::pair<bool,bool>(1,1))) {}

原因是什么？是4.3的bug吗？

原文

Consider:

void f(std::pair<bool,bool> terms = std::pair<bool,bool>(1,1)) {}

gcc 4.4 is ok, gcc 4.3 complains error: expected ',' or '...' before '>' token. The fix is:

void f(std::pair<bool,bool> terms = (std::pair<bool,bool>(1,1))) {}

What's the reason? Is it a bug in 4.3?

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始终不够 2024-11-07 06:59:56

这是一个已知问题。它认为第二个逗号分隔参数声明。这是因为在类定义中，函数默认参数首先仅被标记，然后仅在读取完整类主体后才进行解析。由于它并没有真正解析默认参数，所以它没有注意到逗号实际上是模板参数列表中的逗号。

请参阅http://www.open-std.org/jtc1/sc22/wg21 /docs/cwg_active.html#325 了解有关它的内容。被引用

另一个问题是收集形成默认参数表达式的标记。包含具有多个参数的模板 ID 的默认参数在确定默认参数何时完成时存在困难。考虑一下，
template ;结构 T { 静态 int i;};
C类{
  int Foo(int i = T<1, int>::i);
};
默认参数包含非括号逗号。是否要求将此逗号视为默认参数表达式的一部分，而不是另一个参数声明的开头？要接受它作为默认参数的一部分，需要在 C 完成之前对 T 进行名称查找（以确定“<”是模板参数列表的一部分，而不是小于运算符）。此外，更加病态
D 类 {
  int Foo(int i = T<1, int>::i);
  模板 结构 T {静态 int i;};
};
很难接受。即使 T 是在 Foo 之后声明的，T 也在 Foo 的默认参数表达式的范围内。

This was a known issue. It thinks that the second comma separates parameter declarations. This comes from the fact that in a class definition, function default arguments are first only tokenized, and then only later parsed when the full class body has been read. As it thus doesn't really parse the default argument, it doesn't notice the comma is really a comma within a template argument list instead.

See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#325 for something to read about it. Cited

The other problem is with collecting the tokens that form the default argument expression. Default arguments which contain template-ids with more than one parameter present a difficulty in determining when the default argument finishes. Consider,
template <int A, typename B> struct T { static int i;};
class C {
  int Foo (int i = T<1, int>::i);
};
The default argument contains a non-parenthesized comma. Is it required that this comma is seen as part of the default argument expression and not the beginning of another of argument declaration? To accept this as part of the default argument would require name lookup of T (to determine that the '<' was part of a template argument list and not a less-than operator) before C is complete. Furthermore, the more pathological
class D {
  int Foo (int i = T<1, int>::i);
  template <int A, typename B> struct T {static int i;};
};
would be very hard to accept. Even though T is declared after Foo, T is in scope within Foo's default argument expression.

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