c++ - C++17 不能使用 std::bind 生成 std​​::function

Question

我有一个Register以 astd::function<void(const uint8_t* data, size_t len)>作为参数的函数。我想使用对象的成员函数作为目标。

我发现this question根据它的答案是使用std::bind将第一个参数（隐式this指针）绑定到实际的对象指针，然后将其用作std::function参数。

然而，这在 C++11、C++14 和 C++17 中都不再起作用了吗？

考虑以下测试程序。

#include <iostream>
#include <cstdint>
#include <functional>

void Register(std::function<void(const uint8_t* data, size_t len)> func) {
    //Dummy - directly call into function
    func(nullptr, 0);
}

class TestClass {
public:
    void TestRegister() {
         Register(
                 std::bind(&TestClass::TestTarget, this, std::placeholders::_1)
         );
    }

    void TestTarget(const uint8_t* data, size_t len) {
        (void) data;
        (void) len;
        std::cout << "Hello there" << std::endl;
    }
};


int main() {
    TestClass testObj;
    testObj.TestRegister();
    return 0;
}

当为此编译时-std=c++17会抛出一个相当神秘的错误消息（我不知道它在这里试图说什么Wrong number of arguments for pointer-to-member）：

In file included from /home/max/Documents/TestingFunctions/main.cpp:3:0:
/usr/include/c++/7/functional: In instantiation of ‘struct std::_Bind_check_arity<void (TestClass::*)(const unsigned char*, long unsigned int), TestClass*, const std::_Placeholder<1>&>’:
/usr/include/c++/7/functional:854:12:   required from ‘struct std::_Bind_helper<false, void (TestClass::*)(const unsigned char*, long unsigned int), TestClass*, const std::_Placeholder<1>&>’
/usr/include/c++/7/functional:875:5:   required by substitution of ‘template<class _Func, class ... _BoundArgs> typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type std::bind(_Func&&, _BoundArgs&& ...) [with _Func = void (TestClass::*)(const unsigned char*, long unsigned int); _BoundArgs = {TestClass*, const std::_Placeholder<1>&}]’
/home/max/Documents/TestingFunctions/main.cpp:14:78:   required from here
/usr/include/c++/7/functional:841:7: error: static assertion failed: Wrong number of arguments for pointer-to-member
       static_assert(_Varargs::value
       ^~~~~~~~~~~~~
/home/max/Documents/TestingFunctions/main.cpp: In member function ‘void TestClass::TestRegister()’:
/home/max/Documents/TestingFunctions/main.cpp:14:26: error: could not convert ‘std::bind(_Func&&, _BoundArgs&& ...) [with _Func = void (TestClass::*)(const unsigned char*, long unsigned int); _BoundArgs = {TestClass*, const std::_Placeholder<1>&}; typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type = std::_Bind<void (TestClass::*(TestClass*, std::_Placeholder<1>))(const unsigned char*, long unsigned int)>](((TestClass*)this), std::placeholders::_1)’ from ‘std::_Bind_helper<false, void (TestClass::*)(const unsigned char*, long unsigned int), TestClass*, const std::_Placeholder<1>&>::type {aka std::_Bind<void (TestClass::*(TestClass*, std::_Placeholder<1>))(const unsigned char*, long unsigned int)>}’ to ‘std::function<void(const unsigned char*, long unsigned int)>’
                 std::bind(&TestClass::TestTarget, this, std::placeholders::_1)
                 ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

将函数替换为TestRegister在 lambda 表达式中执行完全相同操作的函数可以编译并运行而不会出现问题。

void TestRegister() {
    Register(
            [this](const uint8_t* data, size_t len) {
                TestTarget(data, len);
            }
    );
}

问题：为什么std::bind链接问题的方法不起作用？是否删除了此功能或者我的代码中有错误？

标签： c++functional-programmingc++17stdbind

Answer 1

您的函数Register需要一个具有两个参数的函数，但您尝试将一个具有一个占位参数的函数传递给它。

void TestRegister() {
     Register(
             std::bind(&TestClass::TestTarget, this, std::placeholders::_1, std::placeholders::_2)
     );
}