C++ 模板参数推导/替换失败，当使用 std::function 和 std::bind 时
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template argument deduction/substitution failed, when using std::function and std::bind
提问by haipeng31
I have a compile error when using std::function in a templated member function, the following code is a simple example:
在模板化成员函数中使用 std::function 时出现编译错误，以下代码是一个简单示例：
#include <functional>
#include <memory>
using std::function;
using std::bind;
using std::shared_ptr;
class Test {
public:
template <typename T>
void setCallback(function<void (T, int)> cb);
};
template <typename T>
void Test::setCallback(function<void (T, int)> cb)
{
// do nothing
}
class TestA {
public:
void testa(int a, int b) { }
};
int main()
{
TestA testA;
Test test;
test.setCallback(bind(&TestA::testa, &testA, std::placeholders::_1, std::placeholders::_2));
return 0;
}
And come with the following compile error:
并出现以下编译错误：
testtemplate.cpp: In function ‘int main()':
testtemplate.cpp:29:92: error: no matching function for call to ‘Test::setCallback(std::_Bind_helper)(int, int), TestA, const std::_Placeholder<1>&, const std::_Placeholder<2>&>::type)'
testtemplate.cpp:29:92: note: candidate is: testtemplate.cpp:10:7: note: template void Test::setCallback(std::function)
testtemplate.cpp:10:7: note: template argument deduction/substitution failed:
testtemplate.cpp:29:92: note: ‘std::_Bind(TestA*, std::_Placeholder<1>, std::_Placeholder<2>)>' is not derived from ‘std::function'
testtemplate.cpp：在函数“int main()”中：
testtemplate.cpp:29:92: 错误：没有匹配的函数调用 'Test::setCallback(std::_Bind_helper)(int, int), TestA, const std::_Placeholder<1>&, const std::_Placeholder <2>&>::type)'
testtemplate.cpp:29:92: 注意: 候选是: testtemplate.cpp:10:7: 注意: 模板 void Test::setCallback(std::function)
testtemplate.cpp:10:7: 注意：模板参数推导/替换失败：
testtemplate.cpp:29:92：注意：'std::_Bind(TestA*, std::_Placeholder<1>, std::_Placeholder<2>)>' 不是从 'std::function' 派生的
I'm using C++11 and g++ 4.7
我正在使用 C++11 和 g++4.7
采纳答案by masoud
To figure out the problem let separate statements:
为了找出问题，让单独的语句：
auto f = bind(&TestA::testa, &testA, _1, _2); // OK
test.setCallback(f); // <<--- Error is here
setCallbackneeds to know type of Tand it can't deduce it from f, so give it a type
setCallback需要知道类型 ofT并且不能从 推断出来f，所以给它一个类型
test.setCallback<TYPE>(f); // TYPE: int, float, a class, ...
回答by dspeyer
You can make type deduction work with some variant of:
您可以使用以下一些变体进行类型推导：
template<typename CALLBACK>
void setCallback(CALLBACK cb) {
typedef CALLBACK::first_argument_type T;
static_assert(is_same_type<CALLBACK,function<void(T,int)>>::value);
...
}
This way CALLBACK can be determined by looking at the argument. It might get into trouble if bind doesn't actually return a std::function but rather something that can be cast as one. I'm not sure.
这样 CALLBACK 就可以通过查看参数来确定。如果 bind 实际上不返回 std::function 而是可以将其转换为一个函数，则可能会遇到麻烦。我不知道。
