How to properly declare a function in C++?

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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?

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

It should be:

string my_function(string message = "");

...

string my_function(string message) { ... }

C - Using Functions, What is C function? Uses of C functions; C function declaration, function call and definition with example program; How to call C functions in a program? Functions allow to structure programs in segments of code to perform individual tasks. In C++, a function is a group of statements that is given a name, and which can be called from some point of the program. The most common syntax to define a function is: - type is the type of the value returned by the function.

C Function with examples, https://www.cs.utah.edu › ~germain › PPS › Topics › C_Language › c_fun A member function of a class is a function that has its definition or its prototype within the class definition like any other variable. It operates on any object of the class of which it is a member, and has access to all the members of a class for that object. Let us take previously defined class to access the members of the class using a

You can use the following solution:

string my_function(string message = "");

...

string my_function(string message)
 {
    your codes here
 }

Function declarations - cppreference.com, C functions must be TYPED (the return type and the type of all parameters specified). Here is the syntax for the function declaration or Prototype: In methods that return boolean, you want to first determine what the value of the result will be when the method returns true, and then use the == operator to evaluate any result you get against the acceptable result. So in your case, you are trying to determine whether to return true or false depending on if the first number is evenly

C Programming - Functions, Functions in c programming with examples: A function is a block of statements, which is used to perform a specific task. Do you find above terms confusing? In C and C++, functions must be declared before the are used. You can declare a function by providing its return value, name, and the types for its arguments. The names of the arguments are optional. A function definition counts as a function declaration. Related articles. Functions tutorial. Jumping into C++, the Cprogramming.com ebook.

Functions in C Programming with examples, A function is a block of code that performs a specific task. C allows you to define functions according to your need. These functions are known as user-defined  Function templates are special functions that can operate with generic types. This allows us to create a function template whose functionality can be adapted to more than one type or class without repeating the entire code for each type. In C++ this can be achieved using template parameters. A template parameter is a special kind of parameter

C User-defined functions, Declaring the variables just before you use them keeps the declaration and use on the same screen without scrolling. ( As a compromise, you can declare the  int * ptrInteger; /*We have put a * operator between int and ptrInteger to create a pointer.*/ Here ptrInteger is a pointer to integer. If you understand this, then logically we should not have any problem in declaring a pointer to a function. So let us first see ..how do we declare a function? For example, Here foo is a function that returns

Comments
  • myfunction and my_function are not the same.
  • Declaration should match definition.
  • The forum appears to be plagued with drive-by downvoting without explanation. I suspect some of the downvoting is done by bots.
  • @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. And then come back with a specific problem you are facing.
  • 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".
  • In this particular case it is just 7 more characters to type. In general case - agree. And don't forget std::string_view.