无法调用其他类-ERR-未解析的外部符号

Question

I am trying to call another function, foo(), in the Raw class from main.cpp but I keep on getting this error and I do not understand what is wrong with my code. I am working in C++, and I am using the QT framework. I am new to this language and framework.

Error: LNK2019: unresolved external symbol "public:void __thiscall RAW::foo(void)" (?foo@Raw@@QAEXXZ) referenced in function_main. File not found:main.obj

main.cpp

#include "raw.h"

using namespace std;


int main(int, char*)
{
    Raw newRaw;
    newRaw.foo();

    return 0;
}

raw.cpp

#include "raw.h"
#include <iostream>

using namespace std;

void foo()
{
    cout << "hi\n";
}

Raw::Raw()
{
    cout << "raw\n";
}

raw.h

#ifndef RAW_H
#define RAW_H

class Raw
{
public:
    Raw();
    void foo();
};

#endif // RAW_H

Answer 1

In raw.cpp you have to define foo like this:

void Raw::foo()
{
    cout << "hi\n";
}

You have to put Raw:: so that the compiler knows that this is the class member function foo and not some other independent function.

Answer 2

As Mihai was saying, you can also define it in the Header file (.h/.hpp), but that is considered bad practice if your class method is complex in any way.

class Raw {
public:
    void foo() {
        cout << "hi\n";
    }
};

Only time you should really ever do this is for extremely simple classes and for methods that are nothing more than getters really.

You should understand the difference between defining and declaring something in C++.

Declaring is to simply make a prototype, for example void doSomething(int); is a valid declaration as it says that the method doSomething takes an int and returns void.

Now, you need to describe what it does. This is the definition when you do void doSomething(int val) { cout << val << endl; } void doSomething(int val) { cout << val << endl; } as you are now describing what to do with that function.

You can make the definition in the header file, as I showed, or in the source file (.c/.cpp) as Mihai showed (which is best practice). You can only make your declarations in the Header file, though.

Answer 3

Ok, this are some alternatives:

//raw.h

#ifndef RAW_H
#define RAW_H

class Raw
{
public:
    Raw();
    void foo(){cout << "raw\n";}
};

or

//raw.h

#ifndef RAW_H
#define RAW_H

class Raw
{
public:
    Raw();
    void foo();
};

Raw::Raw()
{
    cout << "raw\n";
}

In both cases you won't need the implementation in raw.cpp anymore. But as I said before, my first solution is standard c++ practice.