.template（点模板）构建用法 [英] .template (dot-template) construction usage

问题描述

可能重复：

我遇到了一个奇怪的代码段：

I've come across a strange segment of code:

#include <iostream>

template <int N>
struct Collection {
  int data[N];

  Collection() {
    for(int i = 0; i < N; ++i) {
      data[i] = 0;
    }
  };

  void SetValue(int v) {
    for(int i = 0; i < N; ++i) {
      data[i] = v;
    }
  };

  template <int I>
  int GetValue(void) const {
    return data[I];
  };
};

template <int N, int I>
void printElement(Collection<N> const & c) {
  std::cout << c.template GetValue<I>() << std::endl; /// doesn't compile without ".template"
}

int main() {
  Collection<10> myc;
  myc.SetValue(5);
  printElement<10, 2>(myc);
  return 0;
}

不会编译 .template printElement 功能。我从来没有见过这个，我不明白需要什么。试图删除它，我有很多模板相关的编译错误。所以我的问题是什么时候使用这样的结构？是否常见？

It is not compiled without .template keyword in printElement function. I've never seen this before and I don't understand what is needed for. Trying to remove it, I got a lot of template-related compilation errors. So my question is when such a construction is used? Is it common?

推荐答案

GetValue 是一个依赖名称，因此你需要明确告诉编译器 c 是一个函数 template ，而不是一些成员数据。这是为什么你需要写模板关键字来消除歧义。

GetValue is a dependent name, and so you need to explicitly tell the compiler that what follows c is a function template, not some member data. That is why you need to write template keyword to disambiguate this.

没有 / code>关键字，以下

Without template keyword, the following

c.GetValue<I>()  //without template keyword

可以解释为：

//GetValue is interpreted as member data, comparing it with I, using < operator
((c.GetValue) < I) > () //attempting to make it a boolean expression

< 被解释为小于运算符，而> 被解释为大于运算符。以上解释当然不正确，因为它没有意义，因此会导致编译错误。

that is, the < is interpreted as less-than operator, and > is interpreted as greater-than operator. The above interpretation is of course incorrect, as it doesn't make sense, and therefore, would result in compilation error.

有关详细说明，请阅读接受的答案在这里：

For more detail explanation, read the accepted answer here:

• 在哪里和为什么我必须把模板和类型名称

• Where and why do I have to put the "template" and "typename" keywords?

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