请问为什么编译器推断不出这里的 Nontype Template Parameter（非类型模板参数？）呢？

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用的编译器是 clang-700.1.81

#include <iostream>
using namespace std;

template <unsigned N, unsigned M>
void compare(const char p1[N], const char p2[M]) {
  cout << N << ", " << M << endl;
}
int main() {
  compare("hi", "hello");
  return 0;
}

错误信息是：

test.cpp:9:3: error: no matching function for call to 'compare'
  compare("hi", "hello");
  ^~~~~~~
test.cpp:5:6: note: candidate template ignored: couldn't infer template argument
      'N'
void compare(char p1[N], char p2[M]) {
     ^
1 error generated.

请问这里为什么编译器推断不出 N 为 3, M 为 6 呢？
但是如果参数是用引用，那就可以了。

#include <iostream>
using namespace std;

template <unsigned N, unsigned M>
void compare(const char (&p1)[N], const char (&p2)[M]) {
  cout << N << ", " << M << endl;
}
int main() {
  compare("hi", "hello");
  return 0;
}

3 replies • 2016-01-12 11:47:59 +08:00

yangff

Jan 12, 2016

大概是类型?

void fuck(const char p1[3]) {
cout << typeid(p1).name();
}

msg7086

Jan 12, 2016

帮你用 gcc 跑了一下，得出以下错误信息：

test.cpp:5:6: note: candidate: template<unsigned int N, unsigned int M> void compare(const char*, const char*)
void compare(const char p1[N], const char p2[M]) {
^

所以很明显不是引用的时候变成了 const char * 了。

jmc891205

Jan 12, 2016

@msg7086 谢谢！我明白了~