Undefined symbols for architecture x86_64, linker command failed - c++

Question

I just started learning c++ and makefiles. Now I'm stuck.
There seem to be dozens of questions like this, but I can't figure out which answer applies to my situation. This answer is probably obvious. Some clues for where to look and reasons for what I am doing is wrong would be greatly appreciated!
This is my error:
Undefined symbols for architecture x86_64:
"App::init(char const*, int, int, int, int, bool)", referenced from:
_main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [../out/MYAPP] Error 1
This is my code:
main.cpp
#include "App.h"
// our App object
App* a_app = 0;
int main(int argc, char* argv[])
{
a_app = new App();
a_app->init("Hello World", 100, 100, 640, 480, true);
//...etc more code
return 0;
}
App.h
#ifndef __App__
#define __App__
#include <SDL2/SDL.h>
class App
{
public:
App(){}
~App(){}
bool init(const char* title, int xpos, int ypos, int width, int height, bool fullscreen);
//..etc
private:
};
#endif /* defined(__App__) */
App.cpp
#include "App.h"
#include <iostream>
using namespace std;
bool App::init(const char* title, int xpos, int ypos, int width, int height, bool fullscreen)
{
// attempt to initialize SDL
if(SDL_Init(SDL_INIT_EVERYTHING) == 0)
{
cout << "SDL init success \n";
int flags = 0;
if(fullscreen)
{
flags = SDL_WINDOW_FULLSCREEN;
}
// init the window
m_pWindow = SDL_CreateWindow(title, xpos, ypos, width, height, flags);
//...etc
}
else
{
cout << "SDL init fail \n";
return false; // SDL init fail
}
cout << "SDL init success \n";
m_bRunning = true; // everything inited successfully, start the main loop
return true;
}
And finally my makefile
CXX = clang++
SDL = -framework SDL2 -framework SDL2_image
INCLUDES = -I ~/Library/Frameworks/SDL2.framework/Headers -I ~/Library/Frameworks/SDL2_image.framework/Headers
CXXFLAGS = -Wall -c -std=c++11 $(INCLUDES)
LDFLAGS = $(SDL) -F ~/Library/Frameworks/
SRC = src
BUILD = build
OUT = ../out
EXE = $(OUT)/MYAPP
OBJECTS = $(BUILD)/main.o $(BUILD)/App.o
all: $(EXE)
$(EXE): $(OBJECTS) | $(OUT)
$(CXX) $(LDFLAGS) $< -o $#
$(BUILD)/%.o : $(SRC)/%.cpp | $(BUILD)
$(CXX) $(CXXFLAGS) $< -o $#
$(BUILD):
mkdir -p $(BUILD)
$(OUT):
mkdir -p $(OUT)
clean:
rm $(BUILD)/*.o && rm $(EXE)

Answer 1

You are not linking all the object files.
$(CXX) $(LDFLAGS) $< -o $#
should be
$(CXX) $(LDFLAGS) $^ -o $#
since the $< pseudo-variable expands to only the first dependency, which is main.o. That matches with the linker error.
In fact, making this modification alone makes the error go away on my machine.