Undefined reference error on linking - Makefile and includes configured - c++

Question

I have two sources file, main.cpp & functions.cpp, and a header filed main.h, and finally a Makefile:
main.cpp
#include "main.h"
int main()
{
Application game;
game.update();
game.draw();
}
functions.cpp
#include "main.h"
Application::Application()
{
window = SDL_CreateWindow("SDL GAME",
SDL_WINDOWPOS_CENTERED,
SDL_WINDOWPOS_CENTERED,
SCREEN_WIDTH,
SCREEN_HEIGHT, 0);
if(!window)
{
printf("Error: %s\n", SDL_GetError());
}
windowSurface = SDL_GetWindowSurface(window);
if(!windowSurface)
{
printf("Error: %s\n", SDL_GetError());
}
}
Application::~Application()
{
SDL_FreeSurface(windowSurface);
SDL_DestroyWindow(window);
}
void Application::update()
{
bool quit = false;
while(!quit)
{
SDL_Event e;
while(SDL_PollEvent(&e) > 0) //Event queue
{
switch(e.type)
{ //Add events here
case SDL_QUIT:
quit = true;
break;
}
}
//DRAW
Application::draw();
//STOP DRAW
SDL_UpdateWindowSurface(window);
}
}
void Application::draw()
{
SDL_UpdateWindowSurface(window);
}
main.h
#pragma once
#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <stdio.h>
#include <string>
const int SCREEN_WIDTH = 500;
const int SCREEN_HEIGHT = 500;
class Application
{
public:
Application();
~Application();
void update();
void draw();
private:
SDL_Window *window = NULL;
SDL_Surface *windowSurface = NULL;
SDL_Event event;
};
Makefile
.PHONY = all clean
CC = g++
SRCS:= $(wildcard src/*.cpp) # Succesfully grabs all source files
BINS := $(SRCS:%.cpp=%)
LINKERFLAG = -lSDL2 -Isrc
all: ${BINS}
%: %.cpp
${CC} ${LINKERFLAG} $< -o $#.o
%.o: %.cpp
${CC} -o $<.o
clean:
rm -rvf *.o ${BINS}
The error
g++ -lSDL2 -Isrc src/main.cpp -o src/main.o
/usr/lib64/gcc/x86_64-suse-linux/10/../../../../x86_64-suse-linux/bin/ld:
/tmp/ccp2ZmwE.o: in function main': main.cpp:(.text+0x11): undefined reference to Application::Application()'
/usr/lib64/gcc/x86_64-suse-linux/10/../../../../x86_64-suse-linux/bin/ld:
main.cpp:(.text+0x1d): undefined reference to Application::update()' /usr/lib64/gcc/x86_64-suse-linux/10/../../../../x86_64-suse-linux/bin/ld: main.cpp:(.text+0x29): undefined reference to Application::draw()'
/usr/lib64/gcc/x86_64-suse-linux/10/../../../../x86_64-suse-linux/bin/ld:
main.cpp:(.text+0x35): undefined reference to
Application::~Application()' /usr/lib64/gcc/x86_64-suse-linux/10/../../../../x86_64-suse-linux/bin/ld: main.cpp:(.text+0x4b): undefined reference to Application::~Application()'
What I have tried: using -c to compile all then running again without to link, no success. I'm coming to c++ and Make from a python background so it is fairly new to me. What I think is happening is it's trying to link main.cpp before compiling functions.cpp, but I don't know how to go about that figuring that out.

Answer 1

Suppose you have a self-contained source file, foo.cpp.
You could build the executable foo in one step:
g++ foo.cpp -o foo
Or you could build the object file, then build the executable from that:
g++ -c foo.cpp -o foo.o
g++ foo.o -o foo
Now suppose you have two source files, main.cpp and functions.cpp. The code in main.cpp calls a function defined in functions.cpp. If you carelessly try to build the executable from only one file:
g++ main.cpp -o app
the compiler will complain that your code calls a function that has no definition. This is what your makefile does, when it tries to build main:
%: %.cpp
${CC} ${LINKERFLAG} $< -o $#.o
The usual way in a case like this is to build the object files first, then link them:
g++ -c main.cpp -o main.o
g++ -c functions.cpp -o functions.o
g++ main.o functions.o -o name_of_executable
You can do that with a makefile that looks like this:
$(EXEC_NAME): main.o functions.o
${CC} ${LINKERFLAG} $^ -o $#
%.o: %.cpp
${CC} -c $< -o $#
There are some more refinements to make (and you don't even have to write that second rule, Make already knows it), but that should be enough to get you started.