I'm not an experienced C++ programmer and I'm having problems compiling. I've got a Heap class that uses a template:
template <class T>
class Heap
{
public:
Heap(const vector<T>& values);
private:
vector<T> d;
// etc.
};
And then in a separate implementation file:
template <class T>
Heap<T>::Heap(const vector<T>& values)
{
d = values;
for (unsigned int i = d.size()-1; i > 0; i--) Heapify(ParentIndex(i));
}
// ... more implementation code ...
And finally a main.cc file:
int main (int argc, char *argv[])
{
vector<int> in;
unsigned int i;
while (cin >> i) in.push_back(i);
Heap<int> h = Heap<int>(in);
return 0;
}
I get these compile errors:
g++ -Wall -I/opt/local/include -c -o main.o main.cc
g++ -Wall -I/opt/local/include -c -o heap.o heap.cc
g++ -Wall -o heap main.o heap.o
Undefined symbols:
"Heap<int>::Heap(std::vector<int, std::allocator<int> > const&)", referenced from:
_main in main.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [heap] Error 1
Why does this not compile? I think the linker is saying it can't find the constructor, but I know it made the object file.
Templates need to be defined 100% within the header file. If you have your Heap<T> implementation in a .cc / .cpp file that is the problem. Move all of the code to the header file and it should fix your issue.
According to the C++ standard, you can use the export keyword thus:
export template<typename foo>...
However, most compilers don't support this. The C++ FAQ has some more info: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.14
See JaredPar's answer for something that's actually reliable.
Related
This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed 2 years ago.
I am learning c++ and the instructor made a video on how to make classes and functions in multiple files.
I have 3 simple c++ files called "main.cpp", "something.h", and "something.cpp", they are all in the same directory that has no other files. (they are below)
the problem is that the linker is throwing an error message and I really don't know why. (maybe I'm just missing something really obvious)
// main.cpp
#include <iostream>
#include "something.h"
int main(){
int a{2}, b{2};
std::cout << add(a,b) << std::endl;
int arr[5] {1,2,4,8,16};
print_arr(arr, 5);
std::cout << "Hello, world\n";
return 0;
}
// something.h
#ifndef _SOMETHING_H_
#define _SOMETHING_H_
int add(int a, int b);
void print_arr(int* arr, unsigned int size);
#endif // _SOMETHING_H_
// something.cpp
#include "something.h"
#include <iostream>
int add(int a, int b){
return a+b;
}
void print_arr(int* arr, unsigned int size){
std::cout << "{ ";
for (int i = 0; i < size; i++)
std::cout << arr << ' ';
std::cout << '}';
}
the error:
Undefined symbols for architecture x86_64:
"add(int, int)", referenced from:
_main in main-06aa98.o
"print_arr(int*, unsigned int)", referenced from:
_main in main-06aa98.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
At simplest, clang++ -Wall -g something.cpp main.cpp -o main.
You could also compile something.cpp first to create something.o...
clang++ -Wall -g -c something.cpp
...then specify to link with that when compiling main.cpp:
clang++ -Wall -g main.cpp something.o
This last approach scales better, as if you only change main.cpp you can just do the second step without recompiling something.o.
I have looked all over Stack Overflow and other websites about this famous error, and all of them are very specific, and in my case I cannot find a solution. I am making an ncurses application and when i try to compile it, it causes the following error:
Undefined symbols for architecture x86_64:
"NCRS::End()", referenced from:
_main in crspro-85eaaf.o
"NCRS::Start()", referenced from:
_main in crspro-85eaaf.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I compile the code with the following line:
$ clang++ crspro.cpp -lncurses -o crspro
Here is the code:
crspro.cpp
#include "ncrs.h"
int main(int argc, char* argv[]) {
NCRS::Start();
getch();
NCRS::End();
return 0;
}
ncrs.h
#ifndef NCRS_H
#define NCRS_H
#include <ncurses.h>
#include <string>
typedef std::string string;
class NCRS {
private:
static bool __curses_on;
static bool __buffer;
static bool __echo;
static bool __keypad;
public:
static void Start(bool bbuffer=false, bool becho=false, bool bkeypad=false);
static void End();
};
#endif
ncrs.cpp
#include "ncrs.h"
static void NCRS::Start(bool bbuffer=false, bool becho=false, bool bkeypad=false) {
initscr();
if (bbuffer) raw();
if (becho) echo(); else noecho();
if (bkeypad) keypad(stdscr, TRUE); else keypad(stdscr, FALSE);
__buffer = bbuffer;
__echo = becho;
__keypad = bkeypad;
__curses_on = true;
}
static void NCRS::End() { nocbreak(); echo(); keypad(stdscr, FALSE); endwin(); }
I don't have any issues in the code itself as far as I can tell. I have tried even including ncrs.cpp (The horror!!) but I still get the same problems.
Can anyone help with this issue? I've had this problem before with other projects and I've had to abandon them because I couldn't find a solution.
Thanks to anyone who can help!
_
_
EDIT
compile with:
clang++ crspro.cpp ncrs.cpp -lncurses -o crspro
returns error:
Undefined symbols for architecture x86_64:
"NCRS::__curses_on", referenced from:
NCRS::Start(bool, bool, bool) in ncrs-e52041.o
"NCRS::__echo", referenced from:
NCRS::Start(bool, bool, bool) in ncrs-e52041.o
"NCRS::__buffer", referenced from:
NCRS::Start(bool, bool, bool) in ncrs-e52041.o
"NCRS::__keypad", referenced from:
NCRS::Start(bool, bool, bool) in ncrs-e52041.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Your compilation isn't including anything from ncrs.cpp, which is where both NCRS::Start() and NCRS::End() are defined. You probably want
clang++ crspro.cpp ncrs.cpp -lncurses -o crspro
Or if you want to build the object files separately and then link them:
clang++ -c crspro.cpp -c
clang++ -c ncrs.cpp -c
clang++ crspro.o ncrs.o -lncurses -o crspro
Your next error about "NCRS::__curses_on" is because you're using static variables without defining them you need to add
bool NCRS::__curses_on=false;
bool NCRS::__buffer=false;
bool NCRS::__echo=false;
bool NCRS::__keypad=false;
to one of your .cpp files. (presumably ncrs.cpp is the logical place.)
It's probably worth thinking about whether they should be static (and whether the functions should be static too) - they may need to be, but static class variables are essentially global variables, which will often come back to bite you later. They make it harder to understand the flow of the code, and can make multi-threading and testing painful.
This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 9 years ago.
It's the first time that I try to use templates in my functions but I can't seem to make them work. I defined my function in a file called ddc.hpp
#ifndef __DIGITAL_DOWN_CONVERTER_H__
#define __DIGITAL_DOWN_CONVERTER_H__
namespace ddc {
template <class T> void perform_resampling(std::vector<T> &, unsigned int, unsigned int);
}
#endif
and implemented it in ddc.cpp
#include "ddc.hpp"
template <class T>
void ddc::perform_resampling(std::vector<T> &data, unsigned int f1, unsigned int f2) {
// do stuff
}
and here's my main.cpp
#include "ddc.hpp"
int main() {
std::vector<float> v (100000);
ddc::perform_resampling(v, 1000, 10);
return 0;
}
Compiling with gcc (linux) I get the following error:
$ g++ -c ddc.cpp -o ddc.o -Wall -O3 -lm -m64
$ g++ -c main.cpp -o main.o -Wall -O3 -lm -m64
$ g++ ddc.o main.o -o ../bin/resampler
main.o: In function `main':
main.cpp:(.text.startup+0xed): undefined reference to `void ddc::perform_resampling<float>(std::vector<float, std::allocator<float> >&, unsigned int, unsigned int)'
collect2: ld returned 1 exit status
make: *** [../bin/HW_3] Error 1
Am I doing something wrong?
Template definitions need to go with declarations, so everything needs to be in the header file.
You need to put your template implementation in the header too.
You need to place the definition of the template function in a location that is visible to the code that uses it or use explicit template instantiation to ensure the code for the function is generated.
If you do not want to expose the implemention of perform_resampling you can still force the the compiler to explicitly generate the code for it. The following line when placed in ddc.cpp will instruct the compiler to generate code for perform_resampling taking a vector<float> as it's first parameter.
template void ddc::perform_resampling(std::vector<float> &data, unsigned int f1, unsigned int f2);
I was starting to code a library, and decided to do a test, but I'm getting the error in the question title (Mac OSX, gcc-4.7.1):
tlib.cpp:
template <typename T>
T dobra(const T& valor){
return valor*2;
}
tlib.h:
template <typename T>
T dobra(const T& valor);
test2.cpp:
#include "tlib.h"
#include <iostream>
using namespace std;
int main (int argc, char const *argv[])
{
double b = dobra<double>(10);
cout << b << endl;
return 0;
}
Compiling:
no25-89:CPROP canesin$ g++ -dynamiclib -Wall -std=c++11 tlib.cpp -o libdobra.so
no25-89:CPROP canesin$ g++ test2.cpp -Wall -std=c++11 -o test2 -L. -ldobra
Undefined symbols for architecture x86_64:
"double dobra<double>(double const&)", referenced from:
_main in cctLJGqf.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
no25-89:CPROP canesin$
A fact of life in C++ is that you must include a complete implementation of a template in every compilation unit it's used in, or restrict yourself to specific instantitions.
In practice, this means that you either:
Put what you've got in tlib.cpp into tlib.h. This is the most common solution.
Restrict yourself to only ever using (say) dobra<double>, and put an explicit instantiation into tlib.cpp:
template double dobra<double>(const double& valor);
I'm relatively new to C & C++ and stuck at compiling (or should I say linking) for the whole 2 days. Anyone gives me an idea would be appreciated.
Error message and 3 code files are below. These are what I cut down to minimum from I'm actually working on so that you guys can take a better glimpse at.
Env: Ubuntu 10.10, Eclipse Indigo CDT, g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
Error message:
**** Build of configuration Debug for project SceneRec2 ****
make all
Building file: ../src/AdaBoost.cpp
Invoking: GCC C++ Compiler
g++ -I"/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Includes" -I/usr/src/linux-headers-2.6.35-30/arch/um/include/shared -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/AdaBoost.d" -MT"src/AdaBoost.d" -o "src/AdaBoost.o" "../src/AdaBoost.cpp"
Finished building: ../src/AdaBoost.cpp
Building file: ../src/AdaMain.cpp
Invoking: GCC C++ Compiler
g++ -I"/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Includes" -I/usr/src/linux-headers-2.6.35-30/arch/um/include/shared -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/AdaMain.d" -MT"src/AdaMain.d" -o "src/AdaMain.o" "../src/AdaMain.cpp"
../src/AdaMain.cpp: In function ‘int main(int, char**)’:
../src/AdaMain.cpp:6: warning: deprecated conversion from string constant to ‘char*’
Finished building: ../src/AdaMain.cpp
Building target: SceneRec2
Invoking: GCC C++ Linker
g++ -o "SceneRec2" ./src/AdaBoost.o ./src/AdaMain.o
./src/AdaMain.o: In function `main':
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:5: undefined reference to `AdaBoost<double>::AdaBoost()'
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:6: undefined reference to `AdaBoost<double>::readFromFile(char*)'
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:8: undefined reference to `AdaBoost<double>::~AdaBoost()'
/home/ubuntuLove/Documents/workspace_eclipse/SceneRec2/Debug/../src/AdaMain.cpp:8: undefined reference to `AdaBoost<double>::~AdaBoost()'
collect2: ld returned 1 exit status
make: *** [SceneRec2] Error 1
**** Build Finished ****
Note-1. I receive the same result when I execute g++ on terminal.
Note-2. The path of .o files in the argument for linker should be correct (./src/###.o).
AdaBoost.h
#ifndef _ADABOOST_H
#define _ADABOOST_H
#include <iostream>
const double eps = 2.2204e-16;
template<class T>
class AdaBoost
{
int N; //Number of Instances
int D; //Number of Dimensions
int nL; //Number of Learners / Classifiers / Rules
T** fVectors;
int* labels;
void learnRule(int t, double* dist);
double genRule(int t, int* L, double* dist);
public:
//Default Constructor
AdaBoost();
//Constructor
AdaBoost(T** data, int* labels, int n, int d, int nL);
//Train function
void train();
//Test function
void test(double** data, double* pMap);
void test(double** data, double* pMap, int n);
int writeToFile(char* fName);
int readFromFile(char* fName);
//Destructor
~AdaBoost();
};
#endif
AdaBoost.cpp
#include "AdaBoost.h"
#include <fstream>
using namespace std;
template class AdaBoost<double> ;
template<class T>
int AdaBoost<T>::readFromFile(char* fName) {
ifstream inFile;
int temp;
int d, dir;
float thr, wt;
inFile.open(fName);
if (!inFile)
return 0;
inFile >> temp;
this->nL = temp;
int k = 0;
while (!inFile.eof() && k < nL) {
inFile >> d;
inFile >> thr;
inFile >> dir;
inFile >> wt;
k++;
}
inFile.close();
return 1;
}
AdaMain.cpp
#include "AdaBoost.h"
using namespace std;
int main(int argc, char** argv)
{
AdaBoost<double> rdClass;
rdClass.readFromFile("Naerer");
return 0;
}
If you are using explicit instantiation, you have to define the generic version of the member function before instantiating the class:
template<class T>
int AdaBoost<T>::readFromFile(char* fName) {
// ...
}
template class AdaBoost<double>;
However, if you don't have a specific or pressing reason to use explicit instantiation in the first place, go with the other recommendations and define the templates in the header.
You cannot separate template class definition and implementation in different compilation units. In other words, AdaBoost<T> complete implementation should be linked in the same compilation unit that main (where it is used) is.
This is typically fixed by either #including the .cpp file at the end of your .hpp file (if you want to maintain them separate), or just using only the .hpp file implementing the whole class there.
You have multiple problems.
First, you use the unconventional technique of explicit instantiation in a CPP file. As others have pointed out, convention (but nothing more) requires that you put the implementation in the .H file to allow for generic instantiation. You don't have to do this, but if you did, the readfile() error would go away. (As an alternative, put your AdaBoost<double> instantiation after the definition of AdaBoost::readfile.)
Next, you have declared, but not defined, your constructor and destructor. If you wish to use the compiler-provided constructor and destructor you should delete the declarations. If you wish to use your own constructor and dstructor, you should define them.
Best practice is to get rid of AdaBoost.cpp, and modify AdaBoost.h to include the implementation inline in the header. (Note that this best practice is for templated classes; other advice may apply to non-templated classes.)
You need to put the definition of template<class T>
int AdaBoost<T>::readFromFile(char* fName) into AdaBoost.h, and remove AdaBoost.cpp from your build.
It's best to put all template code into the header. C++ linkers are required to eliminate duplicate instantiations of template code so you won't get "multiply defined symbol" errors.
P.S. You should declare the function as template<class T>
int AdaBoost<T>::readFromFile(const char* fName) to get rid of the deprecated conversion from string constant to ‘char*’ warning. The function does not need to alter the filename.