undefined reference to function declared in header and implemented in cpp file

Question

I'm working on some basic C++ code that uses two cpp files(Main.cpp and CarbStore.cpp) and one header(CarbStore.h). Within my header I have declared a function that is later implemented in CarbStore.cpp. When I call the function from my Main.cpp it gives me the error:

Main.cpp:17: undefined reference to `CarbStore::CalcCarbs(unsigned char, unsigned char, unsigned char, float, unsigned int, std::__cxx11::basic_string, std::allocator >) const'

My files contain the following code:

Main.cpp

#include <iostream>
#include <cstdint>
#include <cmath>
#include <ctime>

#include "CarbStore.h"


void CarbCalculator()
{
    CarbStore carb;
    carb.CalcCarbs(10, 11, 12, 0.1, 100, "test");
}

int main(int,char *[])
{
    CarbCalculator();

    std::cout << "Press enter to exit." << std::endl;
    std::cin.get();
}

CarbStore.cpp

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

void CalcCarbs(unsigned char r, unsigned char b, unsigned char g, float bounciness, unsigned int price, std::string manufacturer)
{
    //use values at later state
    return;
}

CarbStore.h

#ifndef CARBSTORE_H
#define CARBSTORE_H
#include <vector>

class CarbStore
{
public:
    void CalcCarbs(unsigned char r, unsigned char b, unsigned char g, float bounciness, unsigned int price, std::string manufacturer) const;
};



#endif

Your definition of `CalcCarbs` is a non-member function. Use `void CarbStore::CalcCarbs() { ... }` to make it a member function. — R Sahu, Nov 26 '18 at 22:50
You wrote a ::CalcCarbs() function, it is not an implementation of the CalcCarbs class constructor. Write CalcCarbs::CalCarbs(...) to get ahead. — Hans Passant, Nov 26 '18 at 22:51
They are both being compiled together in my makefile, hence my confusion. — Oberruk, Nov 26 '18 at 22:53

score 0 · Accepted Answer · answered Nov 26 '18 at 23:58

As already told in the comments, the following

void CalcCarbs(unsigned char r, unsigned char b, unsigned char g, float bounciness, unsigned int price, std::string manufacturer)
{
    //use values at later state
    return;
}

does not implement the member function CalcCarbs of CarbStore, but instead it declares and defines a new free function called CalcCarbs. To implement the member function you need to tell the compiler to which class the function definition should belong. This is done by appending the class name together with a double colon before the function name:

void CarbStore::CalcCarbs(unsigned char r, unsigned char b, unsigned char g, float bounciness, unsigned int price, std::string manufacturer)
{
    //use values at later state
    return;
}

The signature must also match. In CarbStore you declared the function const, but you did not do so in the implementation. To correct it:

void CarbStore::CalcCarbs(unsigned char r, unsigned char b, unsigned char g, float bounciness, unsigned int price, std::string manufacturer) const
{
    //use values at later state
    return;
}

However it is highly unlikely that you really want a const member function with empty return, because such a function can only have a lasting effect by modifying global variables or doing IO.

Furthermore, but unrelated to this particular error message:

In CarbStore you are using a std::string, so you need to #include<string>. On the other hand I don't see any std::vector in it, so the #include<vector> seems unnecessary (as do all the other includes besides iostream in Main.cpp).
The return; at the end of the function body is also point-less for a void function.
If main doesn't use the command line arguments, you can also give it the signature int main().

undefined reference to function declared in header and implemented in cpp file

1 Answers1