问题描述
此代码段(摘自 this question) 用 g++ 编译得很好(如所见)，只要返回类型之前的 template 就在那里.相比之下，VC10 不会编译该代码并出现以下错误:
This snippet (taken from this question) compiles fine with g++ (as seen), so long the template before the return type is there. In contrast, VC10 does not compile that code with the following error:
错误 C2244:'A::getAttr':无法将函数定义与现有声明相匹配
error C2244: 'A::getAttr' : unable to match function definition to an existing declaration
如果我删除 template，VC10 很高兴，但 g++ 会发出这个错误:
If I remove the template, VC10 is happy but g++ screams this error:
错误:用作模板的非模板AttributeType"
注意:使用'A::template AttributeType'表示是模板
error: non-template 'AttributeType' used as template
note: use 'A::template AttributeType' to indicate that it is a template
是不是又是因为VC的两阶段查找坏了还是什么原因?哪个编译器就在这里?我怀疑 g++ 是正确的，因为我对这里需要 template 有一个模糊的记忆，就像分配器内部的 rebind 模板一样.
Is it again because of VC's broken two-phase look-up or what is the cause? Which compiler is right here? I suspect g++ to be correct, as I have a vague memory of template being needed here, like with the rebind template inside of allocators.
编辑:我们有一个赢家:g++/GCC(惊喜...).
Edit: We have a winner: g++/GCC (surprise surprise...).
template <typename T, typename K>
class A {
public:
T t;
K k;
template <int i, int unused = 0>
struct AttributeType{
};
template <int i>
AttributeType<i> getAttr();
};
template <typename T, typename K>
template <int i>
typename A<T, K>::template AttributeType<i> A<T, K>::getAttr() {
// ^^^^^^^^ -- needed or not?
return t;
}
int main(){
A<int,int> a;
}
推荐答案
GCC 是对的.AttributeType 是一个依赖的模板名称，后面跟着尖括号<，所以这里需要关键字template来消除歧义1，让编译器清楚后面跟着的是模板名.该规则在 §14.2/4 中提到:
GCC is right. AttributeType is a dependent template-name which is followed by angle bracket <, so the keyword template is required here to remove the ambiguity1, making it clear to the compiler that what is followed is a template-name. The rule is mentioned in §14.2/4:
当成员模板名称特化出现在 之后.或 ->在后缀表达式中，或之后嵌套名称说明符合格 ID，以及后缀表达式或限定 ID明确依赖于模板参数(14.6.2)，成员模板名称必须加前缀通过关键字模板.否则假设名称命名为非模板.
When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.
1 @Johannes 在这里写了一个很好的解释:
1 @Johannes has written a very good explanation here:
我必须将模板"放在哪里以及为什么要放?和类型名称"关键字?
这篇关于哪个编译器是对的?需要模板化返回类型之前的“模板"?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！