Question

I cannot get this simple piece of code to compile without including the TestClass.cpp file explicitly in my main.cpp file. What am I doing wrong? Thanks in advance!

Here is the code:

TestClass.h

#ifndef TESTCLASS_H_ #define TESTCLASS_H_  class TestClass {     public:         static int foo(); };  #endif 

TestClass.cpp

#include 'TestClass.h'  int TestClass::foo() { return 42; } 

main.cpp

#include <iostream>  #include 'TestClass.h'  using namespace std;  int main() {     cout << TestClass::foo() << endl;      return 0; } 

Here is the error:

g++ main.cpp -o main.app /tmp/ccCjOhpy.o: In function `main': main.cpp:(.text+0x18e): undefined reference to `TestClass::foo()' collect2: ld returned 1 exit status 

Answer 1

Include TestClass.cpp into the commandline, so the linker can find the function definition:

g++ main.cpp TestClass.cpp -o main.app 

Alternatively, compile each to their own object file, then tell the compiler to link them together (it will forward them to the linker)

g++ -c main.cpp -o main.o g++ -c TestClass.cpp -o TestClass.o g++ main.o TestClass.o -o main.app