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astorm

11-03-2005 04:31 PM

Can't use template with std::map

I'm writing a small scripting language. I'm using a global std::map to store variables, which can be of any type. However, I can't do

Code:

template <class T>

std::map< string, T > varTable;

outside a function, and I can't do

Code:

void * varTable;



template <class T>

void makeTable()

{

    map< string, T > tVarTable;

    varTable = &tVarTable;

}

because it complains it can't find the function, and I'd probably have problems with dereferencing varTable. If there's no elegant way to do this, I'll have to create a Scalar struct with a void* value and string type. Is there any better way?

Thanks in advance,
Alek

naf	11-03-2005 04:51 PM

You are a dangerous man :)
I got it to create an inherited class like this:

Code:

#include <map>



template <class T>

class varTable : public std::map< std::string, T >

{

    // Your extensions here?  Maybe you do not want public inheritance?

}; 



int main( int argc, char **argv )

{

    varTable< double > doublesTable;



    return 0;

}

Not sure if this is what you wanted.

astorm

11-03-2005 05:14 PM

Thanks. That would work, but it isn't quite what I wanted. I'd have to create a varTable for every possible type of variable. But hey, that still isn't as clunky as what I had before. I'll go ahead and write the code that way, and change it later if I find a better one. Gives me that adrenaline rush. Thanks again.

ta0kira

11-03-2005 06:55 PM

The problem is that the compiler can't use Koenig Lookup to figure out the type you need since you have no argument; if the function took an argument of type 'T' (or something that uses it, like 'vector <T>'), then the compiler could find out what type you need. Since you have no argument, you need to be explicit with the type in the call:

Code:

makeTable <double> ();

.

FYI: if you DO have an argument, but it can't be used to deduce ALL template parameters, such as

Code:

template <class A, class B> void Function(const A&);

place the deducible template arguments first, and those not deducible last. Then call the function like this:

Code:

char arg;

Function <int> (arg);

The compiler will know 'A' is 'char' and you tell it that 'B' is 'int'.
ta0kira

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