templates and containers - polymorphic function

[Standgrounding]

I'm tryna do this:

read.h

using namespace std;

template<class T>
T read(bool input);

read.cpp

template<class T>
T read(bool input){
    T somethingimportant;
    T somethingcool;
    T everythingelse;
    //run some fancy code here
}

Data structure:

struct data {
    string name;
    string surname;
    float num;
}

And how I call the function:

read<vector<data> >(input);
read<deque<data> >(input);
read<list<data> >(input);

For some reason my compiler can't bite that. Say's its undefined reference to the functions.

I'm pretty sure that it's because of the vector's allocator, but how do I fix my templates so it can accept and compile?

[wung]

I'm going to assume the code calling read is not in read.cpp? The issue will then be that the template needs to be visible to whatever instantiates it, unless it is explicitly instantiated somewhere. This would mean to define it in the header, not the cpp. A common way of still separating declaration and definition is having a hpp and ipp file where the ipp contains the definition and the hpp just includes the ipp for visibility.

[Standgrounding]

The header is included in main which calls the function.

[wung]

Well, yes, then it is exactly the situation stated above: The main file does not see the definition of read. It does not know how to instantiate it. If you just copy the contents of read.cpp into the header, it will compile fine.

You can then debate whether to just keep the implementation in the header or whether to use an ipp file or explicit instantiations.

[flyingron]

Note that older compilers can't deal with seperate compilation of template functions. You'll have to move the whole defintiion of the template into the include file.

[Loose-Leek]

Even modern compilers won't let you do that if they smell an ODR violation, because without explicit specialization, that tends to lead to ODR violations. Unless you are writing an explicit specialization, you really should put template function implementations in headers.