Function template specialization.

msh · 07-21-2009, 11:08 AM

I'm trying to get my head around function template specializations and I just can't understand what am I doing wrong here.

Prototypes and error below.

Can someone explain to me what I'm doing wrong?

Code:

// template
template <typename T>
T findMax(const T arr[], int n);

// specializations for char**
template <>
char* findMax(const char* arr[], int n);

I get the following error from GCC:

Code:

error: template-id 'findMax<>' for 'char* findMax(const char**, int)' does not match any template declaration

**laserlight** · 07-21-2009, 11:19 AM

It should be:

Code:

// specializations for char*
template <>
char* findMax(char* const arr[], int n);

since the const is supposed to be applied to T, hence it is applied to char*, not char. If you really want to specialise for char**, then it should be:

Code:

// specializations for char**
template <>
char** findMax(char** const arr[], int n);

msh · 07-21-2009, 11:30 AM

Ah. I see!

It's different from regular functions, e.g:

Code:

void f(const char arr[]);

Could you elaborate some more why it is so? It seems somewhat unintuitive.

**laserlight** · 07-21-2009, 11:33 AM

Quote:

Originally Posted by msh

It's different from regular functions

Different? If you are talking about what the const in the parameter applies to, then no, that is not true. For the parameter of f, the const applies to the char, not the pointer. Perhaps it would be easier to see if we rewrote it as:

Code:

void f(const char* arr);

or:

Code:

void f(char const* arr);

msh · 07-21-2009, 12:56 PM

Okay. I'm lost now.

That still does not work as I want it to, that is, I want the specialization to process something like this:

Code:

const char* s[5] =
{
    "London",
    "Berlin",
    "Paris",
    "Madrid",
    "Lisbon"
};

Kudose · 07-21-2009, 01:29 PM

You are declaring a pointer array in which the elements are 5 characters(?) long.

Code:

#include<stdlib.h>

int main(){

  const char* cities[10] = { "London", "Paris", "NYC" };

  printf("%s", *(cities + 1)); // prints Paris

  system("PAUSE");
  return 0;
}

msh · 07-21-2009, 01:37 PM

I was under the impression that I'm declaring an array of 5 pointers to string constants.

Code:

int main(void) {
    
    const char* s[5] = 
    {
        "London",
        "Berlin",
        "Paris",
        "Madrid",
        "Lisbon"
    };
    
    for (int i = 0; i < 5; i++) {
        std::cout << s[i] << std::endl;
    }
    
    return 0;
}

tabstop · 07-21-2009, 01:48 PM

Quote:

Originally Posted by msh

I was under the impression that I'm declaring an array of 5 pointers to string constants.

Code:

int main(void) {
    
    const char* s[5] = 
    {
        "London",
        "Berlin",
        "Paris",
        "Madrid",
        "Lisbon"
    };
    
    for (int i = 0; i < 5; i++) {
        std::cout << s[i] << std::endl;
    }
    
    return 0;
}

I'm with you -- five pointers to string constants.

If this is supposed to match your function definition thing, well, it doesn't. The const here refers to the fact that your strings are constants; the const in your function declaration says that the array won't be changed (i.e., it's a char * const arr[]).

Elysia · 07-21-2009, 02:58 PM

Quote:

Originally Posted by msh

I was under the impression that I'm declaring an array of 5 pointers to string constants.

That's right. You're defining an array of 5 elements of type const char* (pointer to const char, not const pointer to char).
(Although your function wants a constant POINTER to char*.)

msh · 07-22-2009, 04:59 AM

Well. I managed to sort it out. Apparently, I was, as they say on the Internet, doing it wrong.

Code:

template <typename T> T findMax(T arr[], int n);

template <> const char* findMax(const char* arr[], int n);

This works as I want it to, albeit I'm still unsure about the whole specialization thing.

Elysia · 07-22-2009, 05:06 AM

As explained, it has nothing to do with specialization and everything to do with you using the improper types.
const char* arr[] -> Arrays of pointers to const char.
const char** arr -> Pointers to pointers to const char (can also be seen as an array of pointers to const char).
char* const arr[] -> Constant array of pointers to char.
char** const arr -> Constant pointers to pointers to char (can also be seen as a constant array of pointers to char).

**CornedBee** · 07-22-2009, 05:25 AM

By the way, usually overloading functions is better than specializing them.

linuxdude · 07-22-2009, 10:44 AM

Quote:

Originally Posted by CornedBee

By the way, usually overloading functions is better than specializing them.

I'm confused by what you mean here? If you have a template, then you will specialize functions. If you don't then you can overload them. Could you be more elaborate?

**laserlight** · 07-22-2009, 10:47 AM

Quote:

Originally Posted by linuxdude

I'm confused by what you mean here? If you have a template, then you will specialize functions. If you don't then you can overload them. Could you be more elaborate?

You can overload even when a function template is involved.

Elysia · 07-22-2009, 10:54 AM

Code:

template<typename T> T func();
template<typename T> T func(T);
template<typename T> T func(T, T);
template<typename T, typename T2> T func();
template<typename T, typename T2> T func(T2);
template<typename T, typename T2> T func(T2, T2);

Overloading...

07-21-2009, 11:08 AM	#1
msh Novice Join Date: Jul 2009 Posts: 32	Function template specialization. I'm trying to get my head around function template specializations and I just can't understand what am I doing wrong here. Prototypes and error below. Can someone explain to me what I'm doing wrong? Code: // template template <typename T> T findMax(const T arr[], int n); // specializations for char** template <> char* findMax(const char* arr[], int n); I get the following error from GCC: Code: error: template-id 'findMax<>' for 'char* findMax(const char**, int)' does not match any template declaration
msh is offline

07-21-2009, 11:19 AM	#2
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 10,365	It should be: Code: // specializations for char* template <> char* findMax(char* const arr[], int n); since the const is supposed to be applied to T, hence it is applied to char, not char. If you really want to specialise for char, then it should be: Code: // specializations for char* template <> char findMax(char const arr[], int n); __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way
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07-21-2009, 11:30 AM	#3
msh Novice Join Date: Jul 2009 Posts: 32	Ah. I see! It's different from regular functions, e.g: Code: void f(const char arr[]); Could you elaborate some more why it is so? It seems somewhat unintuitive.
msh is offline

07-21-2009, 12:56 PM	#5
msh Novice Join Date: Jul 2009 Posts: 32	Okay. I'm lost now. That still does not work as I want it to, that is, I want the specialization to process something like this: Code: const char* s[5] = { "London", "Berlin", "Paris", "Madrid", "Lisbon" };
msh is offline

07-21-2009, 01:29 PM	#6
Kudose Registered User Join Date: Jun 2006 Posts: 72	You are declaring a pointer array in which the elements are 5 characters(?) long. Code: #include<stdlib.h> int main(){ const char* cities[10] = { "London", "Paris", "NYC" }; printf("%s", *(cities + 1)); // prints Paris system("PAUSE"); return 0; }
Kudose is offline

07-21-2009, 01:37 PM	#7
msh Novice Join Date: Jul 2009 Posts: 32	I was under the impression that I'm declaring an array of 5 pointers to string constants. Code: int main(void) { const char* s[5] = { "London", "Berlin", "Paris", "Madrid", "Lisbon" }; for (int i = 0; i < 5; i++) { std::cout << s[i] << std::endl; } return 0; }
msh is offline

07-22-2009, 04:59 AM	#10
msh Novice Join Date: Jul 2009 Posts: 32	Well. I managed to sort it out. Apparently, I was, as they say on the Internet, doing it wrong. Code: template <typename T> T findMax(T arr[], int n); template <> const char* findMax(const char* arr[], int n); This works as I want it to, albeit I'm still unsure about the whole specialization thing.
msh is offline

07-22-2009, 05:25 AM	#12
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,439	By the way, usually overloading functions is better than specializing them. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
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