C++ Function Template, Undefined Symbol for Architecture

Question

Can someone please explain to me why the following won't compile?, and hopefully the obvious thing that I have missed...

functions.hpp:

template<typename T> string vector_tostr(std::vector<T> v);

functions.cpp:

template<typename T> string vector_tostr(std::vector<T> v){
    std::stringstream ss;
    std::string thestring = "";
    if(v.size() > 0){
        ss << "[";
        for(size_t i = 0; i < v.size(); i++){
            if(i != 0)
                ss << " ";
            ss << v[i];
        }
        ss << "]";
        thestring = ss.str();
    }
    return thestring;
}

main.cpp

#include "functions.hpp"
int main(int argc, char *argv[]){
   vector<int> thevector;
   thevector.push_back(1);
   thevector.push_back(2);

   string result = vector_tostr(thevector);
   //I have also tried vector_tostr<int>(thevector)
}

The cryptic error I am getting as follows:

Undefined symbols for architecture x86_64: "std::basic_string, std::allocator > vector_tostr(std::vector >)", referenced from: _main in main.o ld: symbol(s) not found for architecture x86_64 collect2: error: ld returned 1 exit status make: * [main] Error 1

OS ? google search shows it may be issue of not including libraries on apple's OS — vrdhn, May 26 '14 at 03:39
Using Mac os X, compiling with g++-mp-4.8 (need openmp for other stuff) — Nicholas Hamilton, May 26 '14 at 03:40

Dennis · Accepted Answer · 2014-05-26T18:25:02.130

5

You are not allowed to seperate the declaration and definition of a templated function in the same way that you would a normal function (declaration in '.hpp' file, definition in '.cpp' file). There are a couple of ways you can get around that.

You can declare AND define the function in the same place in the header file.

OR

You could try this, in a file called functions.inl:

template<typename T> 
inline string vector_tostr(std::vector<T> v){
    std::stringstream ss;
    std::string thestring = "";
    if(v.size() > 0){
        ss << "[";
        for(size_t i = 0; i < v.size(); i++){
            if(i != 0)
                ss << " ";
            ss << v[i];
        }
        ss << "]";
        thestring = ss.str();
    }
    return thestring;
}

Then, at the end of the header file (functions.hpp), type this in:

#include "functions.inl"

.inl is the file extension for the inline header file. You can use this to seperate the declaration and definition of templated functions.

edited May 26 '14 at 18:25

answered May 26 '14 at 03:42

Dennis

231
2
8

Is `.inl` common? Never heard of it before. Why not just define it in the header? Or name the second "header" a `.h`? – clcto May 26 '14 at 03:54
The `.inl` extension is common primarily in Visual C++. It's considered good programming practice to separate the declaration and definition of a function in C++. – Dennis May 26 '14 at 04:32
2

Yet you start with "You are not allowed to seperate the declaration and definition...". This is quite misleading! – juanchopanza May 26 '14 at 05:52
@juanchopanza Fixed the misleading part of the answer. – Dennis May 26 '14 at 18:26

Rakib · Answer 2 · 2014-05-26T03:50:19.587

Templates are instantiated in compile time. What the compiler does is create one overloaded method for each template argument value that is used in the code. For example, using int and double as template argument will create two overloaded methods from the same definition, only vary in argument type. So compiler must be able to see the definition while compiling. You can do this in several ways

Fully defined in header
Defined in for ex. .impl file and include it in the declaring header
Explicitly instantiated with template parameter in a .cpp file, in that case only the instantiated versions can be used.

score 3 · Answer 3 · answered May 26 '14 at 03:40

3

You need to have the implementation of the function template visible where it's used. Otherwise, the compiler doesn't know how to instantiate the function given a type.

Put the function definition in functions.hpp. There is no use of functions.cpp.

answered May 26 '14 at 03:40

R Sahu

204,454
14
159
270

C++ Function Template, Undefined Symbol for Architecture

3 Answers3

Linked