This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 6 years ago.
I'm trying to learn how to use templated classes. I've created a simple templated class numbers that holds 2 numbers that can be any data type. Then I made a method that returns the bigger number of the two in the object. For some reason I keep getting linker errors though... Here's the errors and code. Not sure what's wrong, Visual Studio didn't underline anything in my code.
They say "unresolved external symbol" if it's too small to read.
templated.h
template <class T>
class numbers {
public:
numbers(T x, T y);
T bigger();
private:
T a, b;
};
templated.cpp
#include "templated.h"
#include <iostream>
using namespace std;
template <class T>
numbers<T>::numbers(T x, T y) {
a = x;
b = y;
}
template <class T>
T numbers<T>::bigger() {
return a > b ? a : b;
}
main.cpp
#include <iostream>
#include "templated.h"
using namespace std;
int main() {
numbers <int>pair(1, 2);
cout << pair.bigger() << endl;
return 0;
}
Thanks for reading!
You're not supposed to put template implementations in a cpp/c file. Move them all to your .h file.
This is because cpp files are supposed to take functions that compile into implementation libraries (like dll files or compiled objects), while template functions have undefined types (yet), whose types get defined at compile time.
You can, however, put specializations of your template functions in the cpp file, where you specify what types you want to include in your compiled objects.
Related
This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 7 years ago.
I have no idea why this is happenning, since I think I have everything properly declared and defined.
I have the following program, designed with templates. It's a simple implementation of a queue, with the member functions "add", "substract" and "print".
I have defined the node for the queue in the fine "nodo_colaypila.h":
#ifndef NODO_COLAYPILA_H
#define NODO_COLAYPILA_H
#include <iostream>
template <class T> class cola;
template <class T> class nodo_colaypila
{
T elem;
nodo_colaypila<T>* sig;
friend class cola<T>;
public:
nodo_colaypila(T, nodo_colaypila<T>*);
};
Then the implementation in "nodo_colaypila.cpp"
#include "nodo_colaypila.h"
#include <iostream>
template <class T> nodo_colaypila<T>::nodo_colaypila(T a, nodo_colaypila<T>* siguiente = NULL)
{
elem = a;
sig = siguiente;//ctor
}
Afterwards, the definition and declaration of the queue template class and its functions:
"cola.h":
#ifndef COLA_H
#define COLA_H
#include "nodo_colaypila.h"
template <class T> class cola
{
nodo_colaypila<T>* ult, pri;
public:
cola<T>();
void anade(T&);
T saca();
void print() const;
virtual ~cola();
};
#endif // COLA_H
"cola.cpp":
#include "cola.h"
#include "nodo_colaypila.h"
#include <iostream>
using namespace std;
template <class T> cola<T>::cola()
{
pri = NULL;
ult = NULL;//ctor
}
template <class T> void cola<T>::anade(T& valor)
{
nodo_colaypila <T> * nuevo;
if (ult)
{
nuevo = new nodo_colaypila<T> (valor);
ult->sig = nuevo;
ult = nuevo;
}
if (!pri)
{
pri = nuevo;
}
}
template <class T> T cola<T>::saca()
{
nodo_colaypila <T> * aux;
T valor;
aux = pri;
if (!aux)
{
return 0;
}
pri = aux->sig;
valor = aux->elem;
delete aux;
if(!pri)
{
ult = NULL;
}
return valor;
}
template <class T> cola<T>::~cola()
{
while(pri)
{
saca();
}//dtor
}
template <class T> void cola<T>::print() const
{
nodo_colaypila <T> * aux;
aux = pri;
while(aux)
{
cout << aux->elem << endl;
aux = aux->sig;
}
}
Then, I have a program to test these functions as follows:
"main.cpp"
#include <iostream>
#include "cola.h"
#include "nodo_colaypila.h"
using namespace std;
int main()
{
float a, b, c;
string d, e, f;
cola<float> flo;
cola<string> str;
a = 3.14;
b = 2.71;
c = 6.02;
flo.anade(a);
flo.anade(b);
flo.anade(c);
flo.print();
cout << endl;
d = "John";
e = "Mark";
f = "Matthew";
str.anade(d);
str.anade(e);
str.anade(f);
cout << endl;
c = flo.saca();
cout << "First In First Out Float: " << c << endl;
cout << endl;
f = str.saca();
cout << "First In First Out String: " << f << endl;
cout << endl;
flo.print();
cout << endl;
str.print();
cout << "Hello world!" << endl;
return 0;
}
But when I build, the compiler throws errors in every instance of the template class:
undefined reference to `cola(float)::cola()'... (it's actually cola'<'float'>'::cola(), but this doesn't let me use it like that.)
And so on. Altogether, 17 warnings, counting the ones for the member functions being called in the program.
Why is this? Those functions and constructors WERE defined. I thought that the compiler could replace the "T" in the template with "float", "string" or whatever; that was the advantage of using templates.
I read somewhere here that I should put the declaration of each function in the header file for some reason. Is that right? And if so, why?
This is a common question in C++ programming. There are two valid answers to this. There are advantages and disadvantages to both answers and your choice will depend on context. The common answer is to put all the implementation in the header file, but there's another approach will will be suitable in some cases. The choice is yours.
The code in a template is merely a 'pattern' known to the compiler. The compiler won't compile the constructors cola<float>::cola(...) and cola<string>::cola(...) until it is forced to do so. And we must ensure that this compilation happens for the constructors at least once in the entire compilation process, or we will get the 'undefined reference' error. (This applies to the other methods of cola<T> also.)
Understanding the problem
The problem is caused by the fact that main.cpp and cola.cpp will be compiled separately first. In main.cpp, the compiler will implicitly instantiate the template classes cola<float> and cola<string> because those particular instantiations are used in main.cpp. The bad news is that the implementations of those member functions are not in main.cpp, nor in any header file included in main.cpp, and therefore the compiler can't include complete versions of those functions in main.o. When compiling cola.cpp, the compiler won't compile those instantiations either, because there are no implicit or explicit instantiations of cola<float> or cola<string>. Remember, when compiling cola.cpp, the compiler has no clue which instantiations will be needed; and we can't expect it to compile for every type in order to ensure this problem never happens! (cola<int>, cola<char>, cola<ostream>, cola< cola<int> > ... and so on ...)
The two answers are:
Tell the compiler, at the end of cola.cpp, which particular template classes will be required, forcing it to compile cola<float> and cola<string>.
Put the implementation of the member functions in a header file that will be included every time any other 'translation unit' (such as main.cpp) uses the template class.
Answer 1: Explicitly instantiate the template, and its member definitions
At the end of cola.cpp, you should add lines explicitly instantiating all the relevant templates, such as
template class cola<float>;
template class cola<string>;
and you add the following two lines at the end of nodo_colaypila.cpp:
template class nodo_colaypila<float>;
template class nodo_colaypila<std :: string>;
This will ensure that, when the compiler is compiling cola.cpp that it will explicitly compile all the code for the cola<float> and cola<string> classes. Similarly, nodo_colaypila.cpp contains the implementations of the nodo_colaypila<...> classes.
In this approach, you should ensure that all the of the implementation is placed into one .cpp file (i.e. one translation unit) and that the explicit instantation is placed after the definition of all the functions (i.e. at the end of the file).
Answer 2: Copy the code into the relevant header file
The common answer is to move all the code from the implementation files cola.cpp and nodo_colaypila.cpp into cola.h and nodo_colaypila.h. In the long run, this is more flexible as it means you can use extra instantiations (e.g. cola<char>) without any more work. But it could mean the same functions are compiled many times, once in each translation unit. This is not a big problem, as the linker will correctly ignore the duplicate implementations. But it might slow down the compilation a little.
Summary
The default answer, used by the STL for example and in most of the code that any of us will write, is to put all the implementations in the header files. But in a more private project, you will have more knowledge and control of which particular template classes will be instantiated. In fact, this 'bug' might be seen as a feature, as it stops users of your code from accidentally using instantiations you have not tested for or planned for ("I know this works for cola<float> and cola<string>, if you want to use something else, tell me first and will can verify it works before enabling it.").
Finally, there are three other minor typos in the code in your question:
You are missing an #endif at the end of nodo_colaypila.h
in cola.h nodo_colaypila<T>* ult, pri; should be nodo_colaypila<T> *ult, *pri; - both are pointers.
nodo_colaypila.cpp: The default parameter should be in the header file nodo_colaypila.h, not in this implementation file.
You will have to define the functions inside your header file.
You cannot separate definition of template functions in to the source file and declarations in to header file.
When a template is used in a way that triggers its intstantation, a compiler needs to see that particular templates definition. This is the reason templates are often defined in the header file in which they are declared.
Reference:
C++03 standard, ยง 14.7.2.4:
The definition of a non-exported function template, a non-exported member function template, or a non-exported member function or static data member of a class template shall be present in every translation unit in which it is explicitly instantiated.
EDIT:
To clarify the discussion on the comments:
Technically, there are three ways to get around this linking problem:
To move the definition to the .h file
Add explicit instantiations in the .cpp file.
#include the .cpp file defining the template at the .cpp file using the template.
Each of them have their pros and cons,
Moving the defintions to header files may increase the code size(modern day compilers can avoid this) but will increase the compilation time for sure.
Using the explicit instantiation approach is moving back on to traditional macro like approach.Another disadvantage is that it is necessary to know which template types are needed by the program. For a simple program this is easy but for complicated program this becomes difficult to determine in advance.
While including cpp files is confusing at the same time shares the problems of both above approaches.
I find first method the easiest to follow and implement and hence advocte using it.
This link explains where you're going wrong:
[35.12] Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
Place the definition of your constructors, destructors methods and whatnot in your header file, and that will correct the problem.
This offers another solution:
How can I avoid linker errors with my template functions?
However this requires you to anticipate how your template will be used and, as a general solution, is counter-intuitive. It does solve the corner case though where you develop a template to be used by some internal mechanism, and you want to police the manner in which it is used.
This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Static member initialization in a class template
(3 answers)
Closed 4 years ago.
Let's say for some reason, I want to have a class template MyTemp with some static data member smDummyVar :
Mytemp.h
#ifndef MY_TEMP_H
#define MY_TEMP_H
template<class T>
class MyTemp{
...
private:
static int smDummyVar;
...
};
#include "MyTemp.cpp"
#endif //MY_TEMP_H
Mytemp.cpp
...
template<class T> int MyTemp<T>::smDummyVar = 0;
...
Since the compiler requires that the definition and declaration of a template be at the same place, so I include MyTemp.cpp in MyTemp.h .
Now: I want to use MyTemp at many places and create objects using the template:
case1.cpp
#include "MyTemp.h"
void dummyfunc1(){
MyTemp<int> myTemp1;
}
case2.cpp
#include "MyTemp.h"
void dummyfunc2(){
MyTemp<int> myTemp2;
}
I won't get any error from the compiler, but I'd get warning from the linker:
"multiple definition for MyTemp<int>::smDummyVar" ... defined in invalid_group(case1.o) ... rejected in favour of symbol defined in ...(case2.o)
Question: how can I get rid of this warning ?
Thanks a lot in advance for your help !
====================================
inspired by one of the answers in this thread Why can templates only be implemented in the header file?
I found the following solution:
i. remove #include "MyTemp.cpp" in MyTemp.h
ii. specialized.h
#include "MyTemp.h"
typedef MyTemp<int> MySpecialized;
iii. specialized.cpp
#include "MyTemp.cpp"
template class MyTemp<int>;
iv. give specialized.cpp to cmake file
v. include specilized.h in case1.cpp and case2.cpp
the "multiple definition" warning issued by the linker will go away.
Thank you guys for helping !
This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 6 years ago.
I'm relatively new to C++ and I feel like I'm in over my head. I'm trying to create a graph structure that can take any kind of data by using templates. Here is c_graph.h
#pragma once
#include <vector>
#include <unordered_map>
#include <unordered_set>
template <class T> class c_graph {
private:
std::unordered_map<T,std::unordered_set<T>> adj_matrix;
public:
'' GRAPH OPERATIONS OMITTED ''
};
template <class M> struct node {
public:
M val;
node() {
}
node(M v) {
val = v;
}
};
I would like to support using data directly (hence the template T on the graph), or wrapping the data in a node struct, which is defined at the bottom. My reason for the node struct is sometimes you want different nodes in the graph to have the same data, which wouldn't work with the unordered_map on the outside of the adjacency matrix without a data wrapper.
However I've run into an issue with the unordered_set class. It doesn't have a hash function for node. I read about this problem online and the solution seems to be something like
namespace std {
template <class M> class hash<node<M>> {
public:
size_t operator()(const node<M> &n) const
{
return reinterpret_cast<size_t>(&n);
}
};
};
I have another .cpp file that is trying to use c_graph<node<char>>
However for the life of me I can't get my code to compile. I've tried placing the hash snippet inside c_graph.h, where I get a bunch of linker errors
error LNK2019: unresolved external symbol "public: void __thiscall c_graph<struct node<char> >::add_node(struct node<char>)"...
and I've tried placing it inside c_graph.cpp, where I get
error C2338: The C++ Standard doesn't provide a hash for this type.
Turns out you can't really separate template function declarations from template function implementations. Linker error when using a template class?
Moving the contents of c_graph.cpp into c_graph.h got rid of the linker errors
This question already has answers here:
Explicit specialization of function templates causes linker error
(2 answers)
Closed 8 years ago.
Having recently discovered templates, I've been trying to get a good grasp on them and came across a usage for them that I'd really like to implement; but having tried multiple things, I keep getting errors.
I'm trying to create a function that can take multiple types of parameters at once in any order.
(I'm using VS 2013.)
What I have right now is an "already defined" error (only when including tem.h in more than one file):
the exact first error (the rest are basically the same just for each test.. overload?):
Error 1 error LNK2005: "void __cdecl test<struct A>(struct A *)" (??$test#UA##$$$V##YAXPAUA###Z) already defined in main.obj c:\Users\User\documents\visual studio 2013\Projects\TemplateTest\TemplateTest\foo.obj TemplateTest
foo is just a .cpp file containing only: #include "tem.h"
tem.h:
#ifndef TEM_H
#define TEM_H
struct A {};
struct B {};
#include <iostream>
template<typename First, typename... Rest>
void test(First *t, Rest&&... args){
test(t);
test(std::forward<Rest>(args)...);
}
template<>
void test<A>(A *val){
std::cout << "Handled A" << std::endl;
}
template<>
void test<B>(B *val){
std::cout << "Handled B" << std::endl;
}
#endif
main.cpp:
#include <iostream>
#include "tem.h"
int main(int argc, char *argv[]){
std::cout << "running test..." << std::endl;
A a;
B b;
test(&a, &b);
return 0;
}
What am I missing that is causing this error?
Edit:
Making each instance of test inline prevents the error, but I don't think that's really the best solution
Function template specializations are not themselves templates, they are normal functions with fancy syntax. Like any other normal function, they should not be defined in header files unless you use the inline keyword.
Either add inline to both specializations, or leave just their declarations in the header and move the definitions to some .cpp file.
The main template definition is OK and should be left in the header intact.
A general advice though is to avoid function template specializations. You can use normal non-template overloads:
void test (A*);
void test (B*);
Figuring if something wasn't broke, I'd break it, I decided to specialize a class I had so that it could be templated between float and double precision automagically.
I have the following [simplified] class declaration:
// Quatcam.h
#pragma once
#include <boost/math/quaternion.hpp>
#include <boost/numeric/ublas/matrix.hpp>
template<typename FloatType>
class QuaternionCamera
{
public:
QuaternionCamera();
void applyTranslation(boost::numeric::ublas::vector<FloatType> translationVector);
boost::numeric::ublas::matrix<FloatType> getTranslationMatrix();
protected:
boost::numeric::ublas::vector<FloatType> m_location;
boost::math::quaternion<FloatType> m_orientation;
};
I have defined the member functions in a .cpp file:
//Quatcam.cpp
#include "Quatcam.h"
using namespace boost::numeric::ublas;
template<typename FloatType>
QuaternionCamera<FloatType>::QuaternionCamera()
: m_location(3),
m_orientation(1,0,0,0)
{
m_location[0] = m_location[1] = m_location[2] = 0;
}
template<typename FloatType>
void QuaternionCamera<FloatType>::applyTranslation(boost::numeric::ublas::vector<FloatType> translationVector)
{
m_location += translationVector;
}
template<typename FloatType>
boost::numeric::ublas::matrix<FloatType> QuaternionCamera<FloatType>::getTranslationMatrix()
{
boost::numeric::ublas::matrix<FloatType> returnMatrix = boost::numeric::ublas::identity_matrix<FloatType>(4,4);
boost::numeric::ublas::vector<FloatType> invTrans = -m_location;
returnMatrix(3,0) = invTrans[0];
returnMatrix(3,1) = invTrans[1];
returnMatrix(3,2) = invTrans[2];
return returnMatrix;
}
This code by itself will happily compile into a .lib or .obj file, but attempting to use the class in situ results in linker errors. Here is my example main.cpp attempting to use the class:
#include "Quatcam.h"
#include <boost/numeric/ublas/io.hpp>
#include <iostream>
int main(int argc, char** argv)
{
QuaternionCamera<float> qcam;
boost::numeric::ublas::vector<float> loc(3);
loc[0] = 0;
loc[1] = 5;
loc[2] = 0;
qcam.applyTranslation(loc);
boost::numeric::ublas::matrix<float> qtm = qcam.getTranslationMatrix();
std::cout << "qtm: "<< qtm << std::endl;
return 0;
}
This code fails to link with an error for missing symbols for getTranslationMatrix and applyTranslation. I assume this is because I haven't technically specified a full specialization of the functions for the type float.
Question(s)
Given that the behavior is the same for any atomic input type (float, double, even int, etc...) and only affects the precision of the answers.
Is there a way to force the compiler to emit specializations for all of them without having to;
move all of the function definitions into the header file, or;
explicitly create specializations for all data types that would presumably involve a lot of copypasta?
Recommended links
Why can templates only be implemented in the header file?
Why do C++ template definitions need to be in the header?
Recommended Practice
Instead of moving the definitions from the .cpp to the header, rename the .cpp to .tpp and add #include "Quatcam.tpp" at the end of Quatcam.h.
This is how you typically split up the template declarations, and their definitions, while still having the definitions available for instantiation.
Note: If you follow this road, you should not compile the .tpp by itself, as you were doing with the .cpp.
Explicit Instantiation
You can explicitly instantiate the templates in question in your .cpp to provide them for the linker, but that requires that you know the exact types that you'd require an instantation of.
This means that if you only explicitly instantiate QuaternionCamera<float>, you'd still get a linker error if main.cpp tries to use QuaternionCamera<double>.
There's no way of forcing instantiation of all "atomic input types", you'll have to write them all out explicitly.
template class QuaternionCamera<float>; // explicit instantiation
template class QuaternionCamera<double>; // etc, etc...
You should put these functions into the header file, not into the .cpp source.
The compiler only creates function instantiations after the template argument deduction is complete. The resulting object file will contain a compiled function for each type that the template was used with.
However, .cpp files are compiled separately. So, when you compile Quatcam.cpp, the compiler doesn't find any instantiations for this type, and doesn't create a function body. This is why you end up with a linker error.
To put it simply, this is how your header should look like:
template<typename T>
class Foo {
void Print();
T data;
};
// If template arguments are specified, function body goes to .cpp
template<>
void Foo<float>::Print();
// Template arguments are incomplete, function body should remain in the header
template<typename T>
void Foo<T>::Print() {
std::cout << data;
}
And this should to the .cpp source:
template<>
void Foo<float>::Print() {
std::cout << floor(data);
}