问题描述
我尝试实现函数f：（std :: function-> int），该函数将1s传递到input_functor中
使用c ++可变参数模板。
让input_functor为g。
例如：
- 如果g为
std::function<int(int,int)>，则f返回g(1,1)。 - 如果g为
std::function<int(int,int,1,1)。
#include <functional>
#include <iostream>
template <typename T,typename... Args>
int apply(std::function<int(T,Args...)> func) {
auto tmp = [func](Args... args) {
return func(1,args...);
};
return apply(tmp);
}
template <typename T>
int apply(std::function<int(T)> func) {
return func(1);
}
int main() {
std::function<int(int,int)> f = [](int a,int b) {
return a + b;
};
std::cout << apply(f) << "\n";
return 0;
}
编译器（clang ++）错误msg是它不能匹配候选对象。
main.cpp:9:12: error: no matching function for call to 'apply'
return apply(tmp);
^~~~~
main.cpp:21:18: note: in instantiation of function template specialization 'apply<int,int>' requested here
std::cout << apply(f) << "\n";
^
main.cpp:5:5: note: candidate template ignored: Could not match 'function<int (type-parameter-0-0,type-parameter-0-1...)>' against
'(lambda at main.cpp:6:16)'
int apply(std::function<int(T,Args...)> func) {
^
main.cpp:13:5: note: candidate template ignored: Could not match 'function<int (type-parameter-0-0)>' against '(lambda at main.cpp:6:16)'
int apply(std::function<int(T)> func) {
^
1 error generated.
解决方法
您有2个问题：
-
定义顺序：
template <typename T> int apply(std::function<int(T)> func) { return func(1); }应放置在递归函数之前，以使其可见并结束递归。
-
lambda不是
std::function，所以不会发生推论template <typename T,typename... Args> int apply(std::function<int(T,Args...)> func) { auto tmp = std::function{[func](Args... args) { // C++17 CTAD return func(1,args...); }}; return apply(tmp); }
Demo C ++ 17
由于限于C ++ 11，因此您可以创建特征以知道需要哪个std::function：
template <typenate T,typename Discarded>
struct always_first
{
using type = T;
};
template <typenate T,typename Discarded> using always_first_t = typename always_first<T,Discarded>::type;
// possibly directly
// template <typenate T,typename Discarded> using always_first_t = T;
// but old compilers might have some issues with that construct as std::void_t
然后
std::function<int(always_first_t<int,Args>...)> tmp = /* your lambda */;