Templates trouble

This is a discussion on Templates trouble within the C++ Programming forums, part of the General Programming Boards category; Hello, everyone! This might seem an easy question to you, and I wouldn't even post it if it wasn't for ...

  1. #1
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    Jul 2005
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    25

    Templates trouble

    Hello, everyone! This might seem an easy question to you, and I wouldn't even post it if it wasn't for my love of clean code. So here goes:

    What I want to do is read some pairs of values from a file, one on a line; for example, I might have a file that looks like this:
    Code:
    john
    7
    paul
    3
    luke
    15
    or like this:
    Code:
    ridley scott
    alien
    stanley cubrick
    odyssey 2001
    steven spielberg
    independence day
    Now I thought I can write one function to read them all, using templates:
    Code:
    template<int>
    int convert(char * buf)
    {
    	return atoi(buf);
    }
    
    template<string>
    string convert(char * buf)
    {
    	return string(buf);
    }
    
    template<class T1, class T2>
    vector<pair<T1, T2> > load_vector(ifstream & f)
    {
    	vector<pair<T1, T2> > v;
    	pair<T1, T2> p;
    
    	char buf[128];
    
    	while(!f.eof())
    	{
    		f.getline(buf, sizeof(buf));
    		p.first = convert<T1>(buf);
    		f.getline(buf, sizeof(buf));
    		p.second = convert<T2>(buf);
    
    		if(!p.first.empty())
    		{
    			v.push_back(p);
    		}
    	}
    
    	return v;
    }
    and I would call (say) load_vector<string, int> and get a nicely structured vector.
    Not.
    The compiler generates an error message, along the lines of
    'convert' : invalid template argument for 'function-parameter', compile-time evaluatable constant expression expected
    , which I don't really understand. Isn't <string, int> constant enough? MSDN didn't help, either.
    Thank you for your time

  2. #2
    Registered User mitakeet's Avatar
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    try something like load_vector<make_pair<string, int> >. not sure of the syntax.

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  3. #3
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    You're not using templates correctly. Maybe something like this:
    Code:
    template<typename T>
    T convert(char * buf);
    
    template<>
    int convert<int>(char * buf)
    {
        return atoi(buf);
    }
    
    template<>
    string convert<string>(char * buf)
    {
        return string(buf);
    }

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