Undefined reference to template class constructor duplicate

Encountering an “undefined reference to” error during compilation, particularly when dealing with template class constructors, is a common yet frustrating experience for C++ developers. This error essentially signals that the linker cannot find the compiled definition of a function or constructor that your code is trying to use. While seemingly straightforward, the root cause can often be hidden within the complexities of template instantiation, separate compilation, and header file organization. Understanding the nuances of how C++ handles templates is crucial to effectively diagnose and resolve these issues. This article will delve into the typical causes of this error, focusing specifically on the context of template class constructors, and provide practical solutions to get your code compiling smoothly. We’ll explore common pitfalls and best practices to avoid this common pitfall in the future.

Understanding the “Undefined Reference To” Error

The “undefined reference to” error is a linker error. The compiler successfully translates your C++ code into object files, but the linker’s job is to combine these object files into an executable. When the linker encounters a call to a function (or, in this case, a template class constructor) for which it cannot find a corresponding definition in any of the provided object files or libraries, it throws this error. For standard functions, the issue is often a simple matter of missing include guards, incorrect library linking, or typos in function names. However, with template class constructors, the error can stem from how templates are instantiated and compiled.

Templates, by their nature, are not compiled directly into machine code. Instead, the compiler generates code only when a specific instantiation of the template is used. For example, if you have a template class MyTemplate, the compiler will only generate code for MyTemplate if you actually use MyTemplate somewhere in your code. This lazy instantiation can lead to problems if the compiler never sees the instantiation it needs during the compilation of a particular object file. This is especially true when the template class constructor is defined in a separate .cpp file from the class declaration, which is generally discouraged but sometimes unavoidable.

One common scenario where this error arises is when the definition of the template class constructor is placed in a .cpp file, and that .cpp file is not included in the compilation process that builds the object file containing the code that uses the constructor. The compiler only sees the declaration of the constructor in the header file but never sees its definition, leading to the linker error. This differs from non-template classes, where the compiler expects to find the definition in a separate compilation unit. Template classes require the definition to be visible wherever the template is instantiated.

Several factors can contribute to the “undefined reference to” error when dealing with template class constructors. Improper template instantiation is a primary culprit. As mentioned earlier, templates are only compiled when they are instantiated. If you declare a template class but never explicitly use a specific instantiation of it (e.g., MyTemplate), the compiler might not generate the necessary code for the constructor, leading to the error. Make sure you are actually creating objects of the specific template types you intend to use.

Separate compilation issues are another frequent cause. Unlike regular classes, template class definitions are typically placed entirely within the header file. Separating the definition into a .cpp file and then including the header file in other translation units often results in the constructor not being compiled for the specific template type used in those other units. This can be resolved by either moving the definition into the header file or explicitly instantiating the template in the .cpp file for each type you intend to use. According to a study by Bjarne Stroustrup, the creator of C++, “Templates achieve their flexibility by being instantiated at compile time, which can sometimes lead to linker errors if not handled correctly.” Source: Stroustrup on Templates

Incorrect include paths can also lead to the problem. If the compiler cannot find the header file containing the template class definition, it will not be able to instantiate the template, resulting in the dreaded “undefined reference to” error. Double-check your project’s include paths and ensure that the directory containing the header file is correctly specified. Furthermore, check that the header file is actually included in the source file where the template class constructor is being used. Finally, be aware of circular dependencies, where two or more header files include each other, which can prevent the compiler from properly processing the template definitions. This can often be fixed by using forward declarations.

Solutions and Best Practices

Resolving the “undefined reference to” error requires careful examination of your code and project structure. One of the most effective solutions is to define the template class constructor directly within the header file. This ensures that the compiler has access to the constructor’s definition whenever the header file is included, allowing it to generate the necessary code for the specific template type being used. This is generally considered the best practice for template classes.

If, for some reason, you need to keep the definition in a separate .cpp file, you must explicitly instantiate the template for each type you intend to use in that .cpp file. This is done by adding lines like template MyTemplate::MyTemplate(); at the end of the .cpp file. This tells the compiler to generate code for the constructor for that specific type. However, this approach can become cumbersome if you have many different template types to support. As noted in “Effective C++” by Scott Meyers, “Explicit template instantiation can be a useful technique, but it should be used judiciously, as it can increase compilation time and code size.” Source: Effective C++

Another approach is to use a technique called “inclusion model.” With the inclusion model, you still define the template class constructor in a .cpp file (often named template_impl.h or similar), but instead of compiling that .cpp file directly, you include it at the end of the header file. This essentially merges the definition into the header file, allowing the compiler to generate the necessary code. This can help keep your code organized while still ensuring that the template definition is available wherever it’s needed. Consider the following steps:

Create your template class declaration in MyTemplate.h.
Create the implementation of the template in MyTemplate_impl.h.
At the end of MyTemplate.h, include MyTemplate_impl.h.

This will ensure the compiler always has access to the template definition. Additionally, regularly clean and rebuild your project. Sometimes, outdated object files can cause linking errors. Cleaning your build directory forces the compiler and linker to rebuild everything from scratch, which can often resolve the issue. Make sure your build system is correctly configured to link all necessary libraries.

Debugging Techniques

When faced with an “undefined reference to” error, systematic debugging is crucial. Start by carefully examining the error message. The error message usually includes the name of the function or constructor that is missing, which can help you narrow down the source of the problem. Check the spelling and capitalization of the function name, as even a minor typo can cause the linker to fail. Use a debugger to step through your code and see where the program is trying to call the missing function. This can help you identify the specific location where the error is occurring.

Use verbose compilation flags to get more detailed information from the compiler and linker. These flags can provide valuable insights into the compilation process and help you identify potential issues with include paths, library linking, and template instantiation. For example, the -v flag in GCC and Clang provides verbose output during compilation. Print out the preprocessor output to see exactly what the compiler is seeing. Preprocessing expands all the includes and macros, which can help you determine if the correct header files are being included and in the right order. You can usually do this with a compiler flag such as -E.

Simplify your code to isolate the problem. Comment out sections of code that are not directly related to the template class constructor and see if the error persists. This can help you identify whether the error is caused by a specific interaction between different parts of your code. Create a minimal example that reproduces the error. This can make it easier to share the problem with others and get help from online forums or communities. Don’t be afraid to ask for help! Online communities and forums are great resources for getting assistance with complex C++ problems. Be sure to provide a clear and concise description of the problem, along with a minimal example that reproduces the error.

FAQ Section

Why does this error only occur with template classes?: Templates are instantiated on demand. Unlike regular classes, the compiler only generates code for a template class when a specific instantiation is used. If the compiler doesn't see the instantiation it needs, it won't generate the code, leading to the "undefined reference to" error.
What is the best way to avoid this error?: The best way to avoid this error is to define the template class constructor directly within the header file. This ensures that the compiler always has access to the constructor's definition.
Can I fix this by explicitly instantiating the template?: Yes, you can explicitly instantiate the template for each type you intend to use. However, this can become cumbersome if you have many different template types to support.
What are some common mistakes that cause this error?: Common mistakes include separating the template class definition into a .cpp file, incorrect include paths, and circular dependencies.

Key Takeaways -------------

Ensure template class definitions, especially constructors, are readily available to the compiler.
Favor defining template class constructors within the header file to avoid linking issues.
Double-check include paths and project settings.
Use debugging techniques to isolate the source of the error and simplify your code.

The “undefined reference to” error when dealing with template class constructors can feel like a significant roadblock. By understanding the underlying causes related to template instantiation and compilation, and by applying the solutions and best practices outlined above, you can effectively diagnose and resolve these issues. Remember to keep your template definitions accessible, use verbose compilation flags to gain insights, and don’t hesitate to leverage debugging techniques to pinpoint the root cause. With a systematic approach and a solid understanding of C++ templates, you can overcome this challenge and continue building robust and efficient code. Explore more about advanced template metaprogramming and SFINAE to deepen your understanding of template usage. If you’re looking for more in-depth knowledge, check out our guide to C++ best practices. Happy coding!

Question & Answer :

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.