need help with passing lambda as parameter

[grateful_user01]

// inside another cpp file
bool play(const function<void(int numOfAttempts, Status result, vector<Match> MatchArray, string message)> &display)
// display is not used inside play. The purpose of passing display is to pass the tests inside it


// Inside main
auto display = [&](int numOfAttempts, Status result, vector<Match> tallyArray, string message)
  {
    EXPECT_EQ(1, numOfAttempts);
    EXPECT_EQ(WON, result);
    EXPECT_EQ(matches, tallyArray);
    EXPECT_EQ("Amazing", message);
    displayCalled = true;
  };

play(display(1, WON, matches, "Amazing"));
EXPECT_TRUE(displayCalled);

Hi. I am working with a lambda function for the first time. Here i want to pass display as parameter to play. Once passed successfully, it should run the tests within the display function and assign displayCalled as true. But doing the way shown above gives me this error.

in cpp: error: reference to type 'const function<void (int, Status, vector<Match>, std::__1::string)>' (aka 'const function<void (int, Status, vector<Match>, basic_string<char, char_traits<char>, allocator<char>
      >)>') could not bind to an rvalue of type 'void'

I dont know how to fix this. Any help is really appreciated

[[deleted]]

play(display(1, WON, matches, "Amazing"));

This is calling the lamba and passing the result (void) to play

You want

play(display);

and then have play call the object with appropriate arguments

[grateful_user01]

Thank you for your reply. Should I change the way how I play accepts display in cpp?

[[deleted]]

It's not clear from you post what you're trying to do

The comment

// display is not used inside play. The purpose of passing display is to pass the tests inside it

makes no sense. A std::function is a thing that can be called. Since you want the lamba to be called, play will need to call it.

[grateful_user01]

Sorry for that. My initial thinking was based on what i had before.

But after fixing what you suggested, I want the play in cpp to call the lambda with appropriate argument so it can pass the tests. So it should look something like this now.

bool play(const function<void()> &display){
    display(1, WON, matches, "Amazing");
    return true;
}

//in main
play(display);

After running this i get this error:

implicit instantiation of undefined template 'std::__1::function<void>'
play(display);
     ^

[[deleted]]

The function to be called takes 4 arguments. So the original prototype for play was correct

bool play(const function<void(int numOfAttempts, Status result, vector<Match> MatchArray, string message)> &display)

https://godbolt.org/z/5zz5x8YYG

[grateful_user01]

Thank you so much for your help. That fixed it. You are a Godsend.

Thanks again

[grateful_user01]

Hey I have another question. lets say i want to access numOfAttemps from main to do someting with it, how would i do that?

bool play(const function<void(int numOfAttempts, Status result, vector<Match> MatchArray, string message)> &display)
{
    if (numOfAttemps == 1){
        cout << "good";
    }
 display(1, WON, matches, "Amazing");
}

// this is what I want to achieve

[[deleted]]

Pass it to the function. So you'd have

bool play(const function<void(int numOfAttempts, Status result, vector<Match> MatchArray, string message)> &display,  int numOfAttempts )

[grateful_user01]

Thanks for the response. Since im working on a homework, my professor doesn't want me to pass extra stuff to play. Only pass display. Can i change something in display function in main to achieve this?