Undefined reference to function in code

Question

I'm learning data structures right now, and I've been trying to figure out this error for over an hour!

In my main, I call:

...
#include "Graph.hpp"
    Graph* g  = new Graph();
    g->addVertex("vertex1");
...

and in my Graph.cpp I have:

...
#include "Graph.hpp"
    Vertex * Graph::addVertex( string name ) {
    ...
    }
...

In my Graph.hpp:

...
class Graph{
    Vertex *addVertex(string);
...
}

I'm getting the error when I compile:

undefined reference to `Graph::addVertex(std::string)'

EDIT:

The makefile:

CC=g++
CXXFLAGS = -std=c++11 -O2 -g - Wall
LDFLAGS= -g

main: main.o
    g++ -o main main.o

main.o: Edge.hpp Graph.hpp Vertex.hpp Graph.cpp

Why is there an inclusion of a cpp file? You can also likely get rid of at least the pointer in `main`, if not the other. — chris, May 04 '14 at 22:53
Should I not do that? I'm still trying to understand how class and header files work — ProgrammerTrying, May 04 '14 at 22:54
Find a [good book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). You should include the header and link with the cpp. — chris, May 04 '14 at 22:54
is `addVertex()` a method of `Graph`, or just a function that is not connected to Graph? From your code snap, it seems like the last is the case. — amit, May 04 '14 at 22:55
Possible duplicate of http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix — πάντα ῥεῖ, May 04 '14 at 22:56
`Vertex * addVertex( string name )` -> `Vertex *Graph::addVertex(string name)` -- you **really really really** need to read a good beginner book. Also, please stop putting spaces inside parentheses, it hurts my eyes. — The Paramagnetic Croissant, May 04 '14 at 22:56
@user3477950, Spaces inside parentheses is a valid coding style. I don't use it myself, but tools do often include an option for it. — chris, May 04 '14 at 22:59
@chris I am aware of the fact that it is an existing coding style. Too bad it is. — The Paramagnetic Croissant, May 04 '14 at 23:01
`Vertex * addVertex( string name ) -> Vertex *Graph::addVertex(string name) ` Didn't work — ProgrammerTrying, May 04 '14 at 23:02
@ProgrammerTrying, Are you sure you recompiled it right, or is it giving a different error? — chris, May 04 '14 at 23:05
Graph has #includes for Edge.hpp and Vertex.hpp addVertex is a function of Graph, to add a vertex object to the graph — ProgrammerTrying, May 04 '14 at 23:08
@ProgrammerTrying You need to do the change (adding `Graph::`) only in the `.cpp` file, not in the header. — Daniel Frey, May 04 '14 at 23:09
@DanielFrey I only changed it in the .cpp file. Edited main post to include Makefile — ProgrammerTrying, May 04 '14 at 23:11
This line, which Daniel showed in a Makefile along with some less useful stuff, is the basis for a solution: `main: main.o Graph.o` — Ben Voigt, May 04 '14 at 23:30

score 0 · Answer 1 · answered May 04 '14 at 23:25

0

You have a missing semicolon on the end of your class definition. This will cause whatever comes next to be treated as a variable declaration of that class type, not whatever it was supposed to be. In such cases, later references to "what it was supposed to be" typically fail as well.

Always start with the first compiler error encountered. Error lists in IDEs often confuse the order, I suggest looking at the actual compiler output.

answered May 04 '14 at 23:25

Ben Voigt

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I think it was the makefile. There is actually a semi colon at the end of my class def in the real code. Thanks! – ProgrammerTrying May 04 '14 at 23:30

Chnossos · Accepted Answer · 2014-05-04T23:45:51.237

Answer : Here :

main: main.o
    g++ -o main main.o

Your executable, main, is not linked with the Graph.o file, thus generating an undefined reference.

Solution :

Here is a simple Makefile that you won't have to modify every time you want to add a new .cpp file to your project :

EXE := a.out # nexecutable name

SRC := $(wildcard *.cpp) # get a list of all the .cpp in the current directory

OBJ := $(SRC:.cpp=.o) # compile every .cpp file into a .o file

CXXFLAGS = -W -Wall -std=c++11 -O2

$(EXE): $(OBJ)
    $(CXX) $(OBJ) -o $(EXE) # $(CXX) contains g++ by default

If you want your Makefile to react to header files modifications too :

EXE := a.out # executable name

SRC := $(wildcard *.cpp) # get a list of all the .cpp in the current directory

OBJ := $(SRC:.cpp=.o) # compile every .cpp file into a .o file

DEP := $(OBJ:.o=.d) # from every .o file create a .d file that will keep track of header files

CXXFLAGS = -W -Wall -std=c++11 -O2
CPPFLAGS = -MMD -MP # let g++ handles the header files dependencies for you

-include $(DEP) # includes the .d files into the Makefile

$(EXE): $(OBJ)
    $(CXX) $(OBJ) -o $(EXE) # $(CXX) contains g++ by default

Undefined reference to function in code

2 Answers2