Why do I need to provide constructor implementation in header? C++

Question

I'm creating an object in main.cpp like this:

#include <string>
#include "class.hpp"

std::string param = "name";

int main(){
     Class object(param);
}

class.cpp:

#include <string>

class Class {
public:
    std::string name;
    Class(std::string name) {
        this->name = name;
    }
};

class.hpp:

#pragma once

class Class {
public:
    std::string name;
    Class(std::string name);
};

However, when doing this I receive:

undefined reference to `Class::Class(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>)'

BUT if I put the constructor implementation in the header like this:

#pragma once

class Class {
public:
    std::string name;
    Class(std::string name) {
        this->name = name;
    }
};

It works. But now my hpp and cpp files are identical except for the include statement I have at the top of class.cpp.

My question is, is this bad practice? It seems redundant and wrong. What am I missing here? Why do I need to put the implementation for a constructor in the header?

"_Why do I need to put the implementation for a constructor in the header?_" You don't. Please show us how are you compiling/linking the code. — Algirdas Preidžius, Feb 28 '20 at 16:20
You're declaring `class` twice in your first example (and probably not compiling the cpp file). Don't do that. (And please don't name a class `class`, it's totally confusing.) — Mat, Feb 28 '20 at 16:22
What compiler is allowing you to use the class keyword as a name for a class? :O — DeducibleSteak, Feb 28 '20 at 16:26
This doesn't compile on clang, gcc and MSVC, because of invalid use of a `class` keyword https://godbolt.org/z/BGoVcL what compiler did you used? — Marek R, Feb 28 '20 at 16:32
See the ODR. https://stackoverflow.com/questions/4192170/what-exactly-is-one-definition-rule-in-c — doug, Feb 28 '20 at 16:33
The real answer here [can be found at this link](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Sam Varshavchik, Feb 28 '20 at 16:40
If should go like this https://wandbox.org/permlink/edeClB0fuyca6K8K — Marek R, Feb 28 '20 at 16:42

Jeka · Answer 1 · 2020-02-28T16:39:54.363

You declared 2 classes with same name, your cpp is completely wrong. It does not contain implementation of constructor declared in hpp file, it declares another class and it's implementation. main.cpp includes hpp file and look for implementation which does not exists.

So at first never use reserved keywords as names! test.hpp:

#pragma once
#include <string>
class Test{
public:
    std::string name;
    Test(std::string name);
};

test.cpp:

#include "test.hpp"

Test::Test(std::string name)
{
    this->name = name;
}

In cpp file you don't have to declare class twice, only constructor implementation should be there

Why do I need to provide constructor implementation in header? C++

1 Answers1