如何通过回调在 C++ 中做 Gtest

Question

I am writing Gtest for the Below Setter method and i am getting error while passing a callback method ClientReceiver from Gtest suite. Below is the code snippet

namespace BaseClient {


class ClientTop : public JsonClient {

public:
    typedef std::function<void(const Json::Value & info)> ReceiverCallBack;


public:
    ClientTop(std::string name, int add);
    bool Setter(const std::string & value, ReceiverCallBack & listener);
    
 };

}

Here is GTest for the Setter Method. I am getting error while calling ClientTop_Obj->Setter(Value,ClientReceiver) for ClientReceiver parameters

void ClientReceiver(const Json::Value & data){
    std::cout<<"Call back received\n";
}


TEST_F(BsrfClientBaseTest,setReceiver) {
std::string value = "Run";
bool ret = ClientTop_Obj->Setter(Value,ClientReceiver);

}

Error Received

error: cannot convert ‘void(const Json::Value&)’ to ‘ReceiverCallBack&’ {aka ‘std::function<void(const Json::Value&)>&’}

Answer 1

You can't bind a non-const reference to a temporary.

Consider this example :

bool func(int & l);

bool foobar() {
    // It doesn't like to bind a temporary to the non-const reference.
    // error: cannot bind non-const lvalue reference of type 'callback_func&' {aka 'std::function<void(const int&)>&'} to an rvalue of type 'callback_func' {aka 'std::function<void(const int&)>'}
    return func(10);
}

You get the exact same error message . So the solution is to not have a temporary so that it can bind to the reference.

#include <functional>

using callback_func = std::function<void(const int & param)>;

class example {
public:
    bool setter(callback_func & l);
};
        
void callback(const int & i)
{
    return;
}

bool foobar() {
    example e;

    // It doesn't like to bind a temporary to the non-const reference.
    // error: cannot bind non-const lvalue reference of type 'callback_func&' {aka 'std::function<void(const int&)>&'} to an rvalue of type 'callback_func' {aka 'std::function<void(const int&)>'}
    // return e.setter(callback_func{&callback});

    // So, provide a non-const for it to bind to.
    callback_func f{&callback};
    return e.setter(f);
}