C++ classes without .cpp files?

Question

I don't want to write a .cpp file for every simple c++ class.

when I write a class definition and declaration in a single .hpp file, the linker complains about multiple definition of member functions which are not implemented inside the body of the class.

So I use templates to get rid of linker complaints:

// log.hpp file
template<typename T>
class log_t {
private:
    int m_cnt = 0;
public:
    void log();
};

template<typename T>
void log_t<T>::log() {
    std::cout << ++m_cnt << std::endl;
}

// some random type (int)
typedef log_t<int> log;

And then I can simply use log class in multiple .cpp files without linker complaints.

Is there something fundamentally wrong with this method?

Edit: even when i use this method will member functions become inline ?

Answer 1

If some class doesn't need to be a template, don't make it a template just to "get rid of the linker's complains". Just use proper coding techniques, like providing inline defintions of member functions:

// log.hpp file

#pragma once

#include <iostream> // For std::cout, std::endl

class log {
private:
    static int m_cnt = 0;
public:
    log();
};

// Note "inline" here:
inline log::log() {
    std::cout << ++m_cnt << std::endl;
}

If you provide the function's body inside the class definition, then there's no need to specify the keyword inline:

// log.hpp file

#pragma once

#include <iostream> // For std::cout, std::endl

class log {
private:
    static int m_cnt = 0;
public:
    // Function body inserted here (no need for "inline"):
    log() {
        std::cout << ++m_cnt << std::endl;        
    }
};

Answer 2

As you want the functions to be defined in headers (which are included in multiple source files, which leads to multiple definitions in your case), you should make the functions inline.

2 options:

• implicit inline - do not separate the function definitions
• explicit inline - manually add inline keyword in front of the definitions

_{Using templates just to "work-around" this error is not a good idea. Use templates only when you need them.}