How to properly declare a function in C++?

Question

I'd like to declare a function in my main.cpp, so that my main function can be the first one in the file. It looks like this:

main.cpp

 #include <iostream>
 #include <string>
 using namespace std;

string my_function();
int main () {
    my_function();
    return 0;
}
string my_function(string message = "") {
    string response;
    cout << message;
    getline(cin,response);
    return response;
}

However, when compiling I get an error:

/usr/bin/ld: /tmp/cco8jyj1.o: in function `main':
main.cpp:(.text+0x1f): undefined reference to `my_function[abi:cxx11]()'
collect2: error: ld returned 1 exit status
[Finished in 1.4s with exit code 1]

What is wrong?

typo: change `string my_function(string message = "");` and `string my_function(string message) { your_logic }` — eyllanesc, Aug 18 '18 at 17:35
The forum appears to be plagued with drive-by downvoting without explanation. I suspect some of the downvoting is done by bots. — Eljay, Aug 18 '18 at 17:50
@PonasM I am not the downvoter but was close to becoming one because: this is basic knowledge and is expected of you to cover that part as SO is not a place where you learn a language. Cover the basics first. A good place to start would be these [C++ books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). And then come back with a specific problem you are facing. — Ron, Aug 18 '18 at 17:58
I also didn't downvote. But the downvote button is for "lack of research". There are hundreds of examples and tutorials on the net showing and explaining "how functions work". — Galik, Aug 18 '18 at 18:13

score 4 · Accepted Answer · answered Aug 18 '18 at 17:35

4

string my_function(); is declared but is not defined.

It should be:

string my_function(string message = "");

...

string my_function(string message) { ... }

answered Aug 18 '18 at 17:35

Evg

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score 1 · Answer 2 · answered Aug 18 '18 at 17:37

1

Better:

Make default parameters in the declaration, but not the definition.
Pass strings by const reference so you don't incur an unnecessary copy of the string.

Updated:

string my_function(const string& message = "");

int main() {
    my_function();
    return 0;
}

string my_function(const string& message) {
    string response;
    cout << message;
    getline(cin, response);
    return response;
}

answered Aug 18 '18 at 17:37

selbie

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In this particular case it is just 7 more characters to type. In general case - agree. And don't forget `std::string_view`. – Evg Aug 18 '18 at 17:41

score 0 · Answer 3 · edited Aug 18 '18 at 21:03

0

You can use the following solution:

string my_function(string message = "");

...

string my_function(string message)
 {
    your codes here
 }

edited Aug 18 '18 at 21:03

Grant Miller

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answered Aug 18 '18 at 17:54

Shani

1
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How to properly declare a function in C++?

3 Answers3