为什么下面的代码会出错?我认为编译器只是在这里选择适当的重载?
#include <iostream>
using std::cout;
using std::endl;
template <typename ToCheckFor>
struct InterfaceCheck {
// used by the constexpr function, the function will pass in a pointer to
// a type with the required types
template <typename _ToCheckFor, void (_ToCheckFor::*) ()>
struct InterfaceCheckImplTag {};
// used to check for the presence of a function print()
// template <typename T>
// static constexpr bool function(__attribute__((unused)) void* ptr) {}
template <typename T>
static constexpr bool function(__attribute__((unused)) void* ptr) {
return false;
}
template <typename T>
static constexpr bool function (__attribute__((unused))
InterfaceCheckImplTag<T, &T::print>* ptr) {
return true;
}
constexpr static const bool value = function<ToCheckFor>(nullptr);
};
struct Something {
void print() { cout << "Something::print()" << endl; }
};
int main() {
cout << InterfaceCheck<Something>::value << endl;
return 0;
}
为什么用省略号替换 void*
参数会使代码按预期工作?所以下面的代码可以按预期工作
#include <iostream>
using std::cout;
using std::endl;
template <typename ToCheckFor>
struct InterfaceCheck {
// used by the constexpr function, the function will pass in a pointer to
// a type with the required types
template <typename _ToCheckFor, void (_ToCheckFor::*) ()>
struct InterfaceCheckImplTag {};
// used to check for the presence of a function print()
// template <typename T>
// static constexpr bool function(__attribute__((unused)) void* ptr) {}
template <typename T>
static constexpr bool function(...) {
return false;
}
template <typename T>
static constexpr bool function (__attribute__((unused))
InterfaceCheckImplTag<T, &T::print>* ptr) {
return true;
}
constexpr static const bool value = function<ToCheckFor>(nullptr);
};
struct Something {
void print() { cout << "Something::print()" << endl; }
};
int main() {
cout << InterfaceCheck<Something>::value << endl;
return 0;
}
最佳答案
Why does the following code cause an error?
函数
重载有两种可行的选择。两者都涉及从提供的参数进行转换,并且两种转换都不比另一种转换更好:
error: call to 'function' is ambiguous
constexpr static const bool value = function<ToCheckFor>(nullptr);
^~~~~~~~~~~~~~~~~~~~
test.cpp:36:13: note: in instantiation of template class 'InterfaceCheck<Something>' requested here
cout << InterfaceCheck<Something>::value << endl;
^
test.cpp:17:27: note: candidate function [with T = Something]
static constexpr bool function(__attribute__((unused)) void* ptr) {
^
test.cpp:21:27: note: candidate function [with T = Something]
static constexpr bool function (__attribute__((unused))
使用 function(...)
进行修复是有效的,因为从任何内容到 ...
的转换始终比其他任何内容都“更差”匹配(但仍然合法) 。一旦你了解了它,这就是一个很棒的技巧。
来自 13.3.3.2 排名隐式转换序列 [over.ics.rank]:
When comparing the basic forms of implicit conversion sequences (as defined in 13.3.3.1)
a standard conversion sequence (13.3.3.1.1) is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and
a user-defined conversion sequence (13.3.3.1.2) is a better conversion sequence than an ellipsis conversion sequence (13.3.3.1.3).
历史
我第一次学到这个技术是从Modern C++ Design ,第 2.7 节。我不确定这是否是它的发明地。但这并不是一个糟糕的猜测。这本书现在已经 15 年了,仍然值得一读。
关于c++ - SFINAE 错误,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/36268560/