Function template specialization?

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02-25-2008
cpjust

Function template specialization?

When compiling this code on VC++ 6.0 it gives me:

Code:

error C2667: 'StringToNum' : none of 2 overload have a best conversion error C2668: 'StringToNum' : ambiguous call to overloaded function

Code:

#include <string> #include <sstream> #include <stdexcept> #include <climits> template <typename E, typename T, typename A, typename N> void StringToNum( const std::basic_string<E, T, A>& str, N& num ) { std::basic_stringstream<E, T, A> ss( str ); if ( !(ss >> num) ) { throw std::invalid_argument( "Error converting string to number!" ); } } template <typename E, typename T, typename A> void StringToNum( const std::basic_string<E, T, A>& str, unsigned char& num ) { std::basic_stringstream<E, T, A> ss( str ); unsigned short snum = 0; if ( !(ss >> snum) || (snum > UCHAR_MAX) ) { throw std::invalid_argument( "Error converting string to number!" ); } num = static_cast<unsigned char>( snum ); } using namespace std; int main() { unsigned char num = 0; wstring str( L"Hello world" ); StringToNum( str, num ); return 0; }

and on Comeau it's quite a bit more verbose...
What's wrong and how can I make StringToNum() work for unsigned char numbers?

When I tried compiling it without the specialization for unsigned chars I got this error:

Code:

error C2679: binary '>>' : no operator defined which takes a right-hand operand of type 'char' (or there is no acceptable conversion) D:\Programming\C++\Test\src\main.cpp(15) : see reference to function template instantiation 'class std::basic_istream<unsigned short,struct std::char_traits<unsigned short> > &__cdecl std::operator >>(class std::basic_istream<unsigned short, struct std::char_traits<unsigned short> > &,unsigned char &)' being compiled
02-25-2008
Mario F.

What function would the compiler choose if on the following template N was a unsigned char?

Code:

template <typename E, typename T, typename A, typename N>
02-25-2008
cpjust

Quote:

Originally Posted by Mario F.

What function would the compiler choose if on the following template N was a unsigned char?

Code:

template <typename E, typename T, typename A, typename N>

Since I provided a function that excplicitely takes an unsigned char, shouldn't it choose that one?
02-25-2008
laserlight

I tested cpjust's original code example with the MinGW port of g++ 3.4.5 and MSVC8, and it compiled without even a warning in both cases. However, it ran with an segfault in both cases too.
02-25-2008
Mario F.

I believe he should do a partial specialization of the first template to take a unsigned char&.

Code:

template <typename E, typename T, typename A> void StringToNum<typename E, typename T, typename A, unsigned char&>( const std::basic_string<E, T, A>& str, unsigned char& num ) {
02-25-2008
cpjust

Quote:

Originally Posted by laserlight

I tested cpjust's original code example with the MinGW port of g++ 3.4.5 and MSVC8, and it compiled without even a warning in both cases. However, it ran with an segfault in both cases too.

Try using Comeau. It gives you pages of errors.
02-25-2008
cpjust

Quote:

Originally Posted by Mario F.

I believe he should do a partial specialization of the first template to take a unsigned char&.

Code:

template <typename E, typename T, typename A> void StringToNum<typename E, typename T, typename A, unsigned char&>( const std::basic_string<E, T, A>& str, unsigned char& num ) {

That didn't work. This is on VC++ 6.0, so maybe that has something to do with it also...
02-25-2008
laserlight

Quote:

I believe he should do a partial specialization of the first template to take a unsigned char&.

From what I understand C++ does not have partial function template specialisation at the moment.

Quote:

Try using Comeau. It gives you pages of errors.

Yes, I did. I wanted to point out an inconsistency here with two other compilers that are reasonably standard conformant.
02-25-2008
Mario F.

Quote:

Originally Posted by cpjust

That didn't work.

Nor it should. I'm sorry. Copyed paste code in a whim and didn't check my post properly. If anything you wouldn't need the arguments declaration. But you have a function template there. Not a class template. So I did even worst, function templates can't be partially specialized.
02-25-2008
cpjust

Quote:

Originally Posted by laserlight

I tested cpjust's original code example with the MinGW port of g++ 3.4.5 and MSVC8, and it compiled without even a warning in both cases. However, it ran with an segfault in both cases too.

Oh, I think I might know why it crashed. Change the string to "255" instead of "Hello world". :D

Anyways, I found a workable, but slightly less desirable solution:

Code:

template <typename E, typename T, typename A, typename N> void StringToNum( const std::basic_string<E, T, A>& str, N& num ) { std::basic_stringstream<E, T, A> ss( str ); long lnum = 0; if ( !(ss >> lnum) || (lnum < std::numeric_limits<N>::min()) || (lnum > std::numeric_limits<N>::max()) ) { throw std::invalid_argument( "Error converting string to number!" ); } num = static_cast<N>( lnum ); }

BTW, I tried using long long int lint = 0; but VC++ 6.0 said I can't say long twice.
I've seen that used in other examples to get 64-bit integers... Is long long standard, or is it a compiler extension?
02-25-2008
Elysia

__int64 would do the trick on Microsoft compilers.
02-25-2008
Mario F.

It's currently an extension on some C++ compilers.
02-25-2008
cpjust

There probably isn't a basic_stringstream overload for operator>>() for 64-bit ints anyways, so maybe I'd better keep it as a long.
02-25-2008
Elysia

Sure, there is.

Code:

std::stringstream s; s << (uint64_t)0; // Works s << (int64_t)0; // Works uint64_t n; s >> n; // Works s >> (int64_t)n; // Doesn't work! :(

At least on Microsoft's compilers.
02-25-2008
cpjust

Quote:

Originally Posted by Elysia

Sure, there is.

Code:

std::stringstream s; s << (uint64_t)0; // Works s << (int64_t)0; // Works uint64_t n; s >> n; // Works s >> (int64_t)n; // Doesn't work! :(

At least on Microsoft's compilers.

I think they key phrase there is "At least on Microsoft's compilers".
I tried using a long long with stringstreams on Comeau and it didn't have an operator>>() for long long.
02-25-2008
CornedBee

It's an implementation issue. Large integer types are still compiler extensions, so it's up to the compiler if it extends the standard library, too.
02-25-2008
brewbuck

Quote:

Originally Posted by laserlight

From what I understand C++ does not have partial function template specialisation at the moment.

Nor does it require it, since the same effects can be achieved with overloading.
02-25-2008
cpjust

Quote:

Originally Posted by brewbuck

Nor does it require it, since the same effects can be achieved with overloading.

Unless you're using VC++ 6.0 :p