问题
Is it possible to call function objects created with std::bind using std::async. The following code fails to compile:
#include <iostream>
#include <future>
#include <functional>
using namespace std;
class Adder {
public:
int add(int x, int y) {
return x + y;
}
};
int main(int argc, const char * argv[])
{
Adder a;
function<int(int, int)> sumFunc = bind(&Adder::add, &a, 1, 2);
auto future = async(launch::async, sumFunc); // ERROR HERE
cout << future.get();
return 0;
}
The error is:
No matching function for call to 'async': Candidate template ignored: substitution failure [with Fp = std::_1::function &, Args = <>]: no type named 'type' in 'std::_1::__invoke_of, >
Is it just not possible to use async with std::function objects or am I doing something wrong?
(This is being compiled using Xcode 5 with the Apple LLVM 5.0 compiler)
回答1:
Is it possible to call function objects created with
std::bind
usingstd::async
Yes, you can call any functor, as long as you provide the right number of arguments.
am I doing something wrong?
You're converting the bound function, which takes no arguments, to a function<int(int,int)>
, which takes (and ignores) two arguments; then trying to launch that with no arguments.
You could specify the correct signature:
function<int()> sumFunc = bind(&Adder::add, &a, 1, 2);
or avoid the overhead of creating a function
:
auto sumFunc = bind(&Adder::add, &a, 1, 2);
or not bother with bind
at all:
auto future = async(launch::async, &Adder::add, &a, 1, 2);
or use a lambda:
auto future = async(launch::async, []{return a.add(1,2);});
来源:https://stackoverflow.com/questions/19079179/can-stdasync-call-stdfunction-objects