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converting int to const char*

Started by Lith Nov 7, 2009 at 3:13 PM 21 replies 5.4k views

Original Post

Nov 7, 2009 at 3:13 PM

how would i convert an int to const char* i need to do this because im using the SDL_TTF extention library to render text. i have my to scores(int) and i have my two surfaces(SDL_Surface*) the

TTF_RenderText_Solid

function needs const char* EDIT: the error im getting is this:

main.cpp(143) : error C2664: 'TTF_RenderText_Solid' : cannot convert parameter 2 from 'int' to 'const char *'

with this code:

lscore_spr = TTF_RenderText_Solid(font, lscore, textColor);

rip-off

Nov 7, 2009 at 3:20 PM

In C++, we use boost::lexical_cast, or std::stringstream:

#include <string>#include <boost/lexical_cast.hpp>// ...std::string text = boost::lexical_cast<std::string>(lscore);lscore_spr = TTF_RenderText_Solid(font, text.c_str(), textColor);

Or:

#include <string>#include <sstream>// ...std::stringstream stream;stream << lscore;std::string text = stream.str();lscore_spr = TTF_RenderText_Solid(font, text.c_str(), textColor);

Lith

Nov 7, 2009 at 3:52 PM

Thanks

But theres one thing:
i have to draw two different scores(pong game)

this is my code:

//render scores//leftstream << lscore;text = stream.str();lscore_spr = TTF_RenderText_Solid(font, text.c_str(), textColor);text.c_str() = "";//rightstream << rscore;text = stream.str();rscore_spr = TTF_RenderText_Solid(font, text.c_str(), textColor);text.c_str() = "";draww(40,40,lscore_spr,screen);draww(40,600,rscore_spr,screen);

it just prints one 0 after the other:
00000000000000000
the two score vars(lscore and rscore) are set to 0

i tryed to understand the problem more by using a log file to log what text.c_str() was every time i used it. It came up with something like this:

000000000000000000000000000000000000

so i thought that this:

text = stream.str();

was adding to it

so i tyed to set it to NULL after it got the string i needed to use
this is the error i got:

main.cpp(150) : error C2106: '=' : left operand must be l-value

rip-off

Nov 7, 2009 at 4:01 PM

The simplest solution is to create a new stringstream every frame, and for each score variable. For example, make a function:

std::string toString(int number){    std::stringstream stream;    stream << number;    return stream.str();}SDL_Surface *renderNumber(TTF_Font *font, int number, SDL_Color colour){    return TTF_RenderText_Solid(font, toString(number).c_str(), colour);}// ...lscore_spr = renderNumber(font, lscore, textColor);rscore_spr = renderNumber(font, rscore, textColor);// ...

Lith

Nov 7, 2009 at 4:10 PM

thankyou, but...

the compiler is complaining about missing

in my classes now
EDIT: btw it want doing that before i tryed your code

Zahlman

Nov 7, 2009 at 4:58 PM

Quote:
Original post by Lith
so i thought that this:
text = stream.str();
was adding to it

There are two conceptual problems here.

1) This:

text.c_str() = "";

does nothing useful. The .c_str() member function returns a pointer, after all, so this is just taking the copy of the string's internal pointer and assigning it to point at an empty string. This has no effect on the text pointed at by the string.

This is deliberate: you are not supposed to be able to re-point the string's internal pointer. The string is responsible for keeping track of that pointer, not you. If you want to clear out the string, assign to the string.

2) But even then, assigning to the string would have no effect on the stream, because the string 'text' is, again, a copy of the stream's internal string object.

Rip-off's code gets around this by re-creating the stream from scratch every time (via calling the function). Notice that he doesn't reset the contents of anything afterward; there is no need to, because the stream is thrown away, and the string can also be thrown away once you've passed its .c_str() to the rendering function.

Quote:
the compiler is complaining about missing ; in my classes now

That's because there's a ; missing somewhere. Error messages include line numbers, which should point you to the right part of the code. You're supposed to be able, by now, to read code and figure out where simple things like this have gone wrong. This kind of problem-solving is more basic than doing stuff like rendering text with SDL, so you should make sure you can do it yourself first.

Lith

Nov 8, 2009 at 5:13 AM

it said that im missing ; 's on this line:

SDL_Surface* LoadImg(blah blah blah);

its saying:
\func.cpp(10) : error C2143: syntax error : missing ';' before '*'

after some google searches i found out that theres proberly a missing ; somewhere in a different file, even though it said it was missing in a different file.

I found out that i was missing a ; in a header file

Thanks for explaining it to me

stephe

Nov 8, 2009 at 5:48 AM

This may be useful: itoa()

Zakwayda

Nov 8, 2009 at 9:45 AM

Quote:
This may be useful: itoa()

Is there any particular reason you would recommend itoa() over the previous suggestions, lexical_cast and stringstream? (Note that itoa() is non-standard, and may not even be available to the OP.)

dj3hut1

Nov 8, 2009 at 12:04 PM

Hello Lith,

you can also use sprintf ( from ).

char buf[80];sprintf( buf, "%d", number );

stephe

Nov 8, 2009 at 12:15 PM

Quote:
Original post by jyk
Quote:
This may be useful: itoa()
Is there any particular reason you would recommend itoa() over the previous suggestions, lexical_cast and stringstream? (Note that itoa() is non-standard, and may not even be available to the OP.)

Yes, but it is only alternative, I didnt say it is better. I think effect is same and easy to use, if available.. in every way, you are right, itoa() is non standard

SiCrane

Nov 8, 2009 at 12:38 PM

Quote:
Original post by stephe
I didnt say it is better. I think effect is same and easy to use, if available.. in every way, you are right, itoa() is non standard

Whether or not it is easy to use depends highly on which version of itoa() is provided, and itoa() is one of the more severely non-standard functions out there. I've run across at least five different function signatures for itoa:

char * itoa(int, char *, int); // you supply the bufferchar * itoa(int, int, char *); // you supply the bufferchar * itoa(int, char *);      // you supply the bufferchar * itoa(int, int);         // you need to free the pointerchar * itoa(int);              // you need to free the pointer

And this ignores the fact that itoa() may in fact be spelled _itoa() depending on your platform. In addition to the variants I've run across myself, I've been told about one more that apparently appeared in very early Unix machines:

void itoa(int input, void (*subr)(char))

This calls the subr function for each character in the resulting string. I believe most people would agree that this would not fall in the "easy to use" category.

leeor_net

Nov 9, 2009 at 2:47 AM

I haven't seen this suggested but this may be too simple:

stringstream myString;myString << someInteger;myString.str().c_str();

EDIT:

Whoops... missed the first reply... :S

[Edited by - leeor_net on March 6, 2010 2:47:17 AM]

mattd

Nov 9, 2009 at 2:55 AM

Check post #2? :)

phresnel

Nov 9, 2009 at 2:56 AM

Quote:
Original post by leeor_net
I haven't seen this suggested but this may be too simple:
stringstream myString;myString << someInteger;myString.str().c_str();

It has been suggested in the first answer given to the OP, which was very concise.

Quote:
Original post by dj3hut1
Hello Lith,

you can also use sprintf ( from ).

char buf[80];sprintf( buf, "%d", number );

Possible undefined behaviour if a compiler defines an int to be 34 or more bytes long. Because of that, we use sooner mentioned methods in C++.

dclyde

Nov 9, 2009 at 4:56 AM

Quote:
Original post by phresnel
Quote:
Original post by dj3hut1
Hello Lith,

you can also use sprintf ( from ).

char buf[80];sprintf( buf, "%d", number );
Possible undefined behaviour if a compiler defines an int to be 34 or more bytes long. Because of that, we use sooner mentioned methods in C++.

Which when it comes down to it that implementation ends up using sprintf or sprintf_s just with a little bit of a smarter buffer size declaration somehow relative to _MAX_INT_DIG from yvals.h

phresnel

Nov 9, 2009 at 5:50 AM

Quote:
Original post by dclyde
Quote:
Original post by phresnel
Quote:
Original post by dj3hut1
Hello Lith,

you can also use sprintf ( from ).

char buf[80];sprintf( buf, "%d", number );
Possible undefined behaviour if a compiler defines an int to be 34 or more bytes long. Because of that, we use sooner mentioned methods in C++.
Which when it comes down to it that implementation ends up using sprintf or sprintf_s just with a little bit of a smarter buffer size declaration somehow relative to _MAX_INT_DIG from yvals.h

_MAX_INT_DIG and yvals.h are non-standard extensions. Also note that _MAX_INT_DIG fails to communicate the base for which an int yields that maximum number of digits.

If for whatever reason you really must use C string functions, then do it at least generic:

#include <climits>// Make *_MAX available for template code.template <typename T> struct get_val_max;template <> struct get_val_max<char> { enum { value = CHAR_MAX }; };template <> struct get_val_max<signed char> { enum { value = SCHAR_MAX }; };template <> struct get_val_max<unsigned char> { enum { value = UCHAR_MAX }; };template <> struct get_val_max<short> { enum { value = SHRT_MAX }; };template <> struct get_val_max<unsigned short> { enum { value = USHRT_MAX }; };template <> struct get_val_max<int> { enum { value = INT_MAX }; };template <> struct get_val_max<unsigned int> { enum { value = UINT_MAX }; };template <> struct get_val_max<long> { enum { value = LONG_MAX }; };template <> struct get_val_max<unsigned long> { enum { value = ULONG_MAX }; };#ifdef LLONG_MAXtemplate <> struct get_val_max<long long> { enum { value = LLONG_MAX }; };template <> struct get_val_max<unsigned long long> { enum { value = ULLONG_MAX }; };#endif

// Check signedness of a type.template <typename T> struct is_signed { enum { value = 1 }; };template <> struct is_signed <char> { enum { value = CHAR_MIN < 0 }; };template <> struct is_signed <unsigned char> { enum { value = 0 }; };template <> struct is_signed <unsigned short> { enum { value = 0 }; };template <> struct is_signed <unsigned int> { enum { value = 0 }; };template <> struct is_signed <unsigned long> { enum { value = 0 }; };#ifdef LLONG_MAXtemplate <> struct is_signed <unsigned long long> { enum { value = 0 }; };#endif

// Recurse and divide by base until zero is reachedtemplate <typename T, T base, T val, bool recurse=val!=0>struct get_dig_max_rec {        enum { value = 1 + get_dig_max_rec<T, base, val/base>::value };};template <typename T, T base, T val> struct get_dig_max_rec<T,base,val,false> {        enum { value = 0 };};

Replacement for non-standard and non-generic _MAX_INT_DIG:

// Get number of digits needed to represent any value of T in basetemplate <typename T, int base> struct get_dig_max {        enum {                value = is_signed<T>::value                      + get_dig_max_rec<T, base, get_val_max<T>::value>::value        };};// some typedefstemplate <typename T> struct get_base10_dig_max : get_dig_max<T,10> {};template <typename T> struct get_base16_dig_max : get_dig_max<T,16> {};template <typename T> struct get_base2_dig_max : get_dig_max<T,2> {};

Test:

#include <iostream>int main () {        std::cout << get_base10_dig_max<char>::value << '\n';        std::cout << get_base10_dig_max<signed char>::value << '\n';        std::cout << get_base10_dig_max<unsigned char>::value << '\n';        std::cout << get_base10_dig_max<unsigned int>::value << '\n';        std::cout << get_base10_dig_max<int>::value << '\n';        // etc.}

So in case of dj3hut1's code:

char buf[get_base10_dig_max<int>::value + 1];

or later when C++0x is here

char buf[get_base10_dig_max<decltype(your_var)>::value + 1];

That way, the code does not and won't yield undefined behaviour, and does not waste memory at the same time.

edit: fixed is_signed, thanks mattd.

[Edited by - phresnel on November 9, 2009 10:50:23 AM]

mattd

Nov 9, 2009 at 6:03 AM

I think you got the is_signed specializations for unsigned types the wrong way around.

phresnel

Nov 9, 2009 at 7:39 AM

Quote:
Original post by mattd
I think you got the is_signed specializations for unsigned types the wrong way around.

Much appreciated :)

Antheus

Nov 9, 2009 at 8:54 AM

I would do something like this:

class ScoreText {public:	ScoreText(int x, int y, TTF_Font * font, SDL_Color color)		: current_score(0)		, font(font)		, cache(NULL)		, color(color)	{		dst.x = x;		dst.y = y;	}	~ScoreText() {		clear_cache();			}	void update(int new_score) {		if (current_score == new_score) return;		current_score = new_score;		clear_cache();	}	void set_position(int x, int y) {		dst.x = x;		dst.y = y;	}	bool render(SDL_Surface * s) {		SDL_Surface * text = get_cached();		if (text != NULL) {			int result = SDL_BlitSurface(text, NULL, s, &dest);			if (result == -2) clear_cache();		}		return result == 0;	}private:	void clear_cache() {		if (cache != NULL) SDL_FreeSurface(cache);		cache = NULL;	}	SDL_Surface * get_cached() {		if (cache == NULL) {			char buf[40];			sprintf(buf, "%d", current_score);			cache = TTF_RenderText_Solid(font, buf, color);		}		return cache;	}	int current_score;	SDL_Rect dest;	SDL_Surface * cache;	SDL_Color color;};

sprintf is ideal choice. We have a well known range, it avoids potential heap allocations, and doesn't require third-party libraries and SDL is a C library, so some 'print_score()' function could be provided as straight C function. In addition, it works on C strings, same as SDL, so there is no need to bring in std::string.

For more generic printing, the alternatives were listed above. Stringstream is a must when several things need to be printed, lexical_cast can be used to work with std::string. printf in general is very clumsy when dealing with straight user input simply due to manual memory management - but this is not the case here.

Good code is primarily about choosing proper compromises. Unless any other constraints are given, stringstream would be generic choice. But this is a common problem today - massive over-engineering without any critical thinking about the actual problem to be solved.

The task here is to Print Score In Pong Game. Score will not be represented with 272 bits. It will probably be 0 to 10.

Instead, rather than writing 500 lines of code to Print a freeking int, going through all the SDL documentation to properly handle all the return codes would be much more productive. When must a surface be freed, what happens if surface is lost (I don't handle this case thoroughly above), which encoding to use, etc... 999:1 chance says that someone will forget to call FreeSurface and leak memory long before their code breaks due to 272-bit integer stack buffer overflow.

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