Append an int to char*

Question

How would you append an integer to a char* in c++?

score 29 · Accepted Answer · edited Aug 18 '17 at 15:52

29

First convert the int to a char* using sprintf():

char integer_string[32];
int integer = 1234;

sprintf(integer_string, "%d", integer);

Then to append it to your other char*, use strcat():

char other_string[64] = "Integer: "; // make sure you allocate enough space to append the other string

strcat(other_string, integer_string); // other_string now contains "Integer: 1234"

edited Aug 18 '17 at 15:52

Vadim Kotov

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answered Dec 07 '08 at 02:40

Paige Ruten

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You have a buffer overflow vulnerability on your hands if sizeof(int) > 4. – Tom Dec 07 '08 at 02:47
@Tom: I'm not used to using the int type (I always use types like u8, u16, u32, etc.) so I didn't think of that... I will change the size of the string then. – Paige Ruten Dec 07 '08 at 03:28
Should use snprintf and strncat, just to be safe. – Brian C. Lane Dec 07 '08 at 03:55
why not do it all via the sprintf?? snprintf(other_string, 64, "Interger: %d", integer); – Lodle Dec 07 '08 at 03:58
Instead of using a fixed constant size for the string, you might want to use something like +sizeof(int)*3+1 (3 = ceil(8*log10(2)), 1 for '-'). This should always work (also in case of 128-bit ints etc.) and also avoids unnecessarily large allocations (probably not an issue). – mweerden Dec 07 '08 at 05:30
For added portability, multiply that factor 3 by CHAR_BITS/8. – MSalters Dec 08 '08 at 10:08
This is quite old. I tried and it show 'unsafe to use' on strcat and sprintf. Any latest answer would be good. – Leo May 04 '22 at 01:24

score 10 · Answer 2 · edited Aug 18 '17 at 15:52

10

You could also use stringstreams.

char *theString = "Some string";
int theInt = 5;
stringstream ss;
ss << theString << theInt;

The string can then be accessed using ss.str();

edited Aug 18 '17 at 15:52

Vadim Kotov

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answered Dec 07 '08 at 02:45

Sydius

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That returns string instead of char * – vozman Mar 02 '22 at 14:29

score 4 · Answer 3 · answered Dec 07 '08 at 02:50

Something like:

width = floor(log10(num))+1;
result = malloc(strlen(str)+len));
sprintf(result, "%s%*d", str, width, num);

You could simplify len by using the maximum length for an integer on your system.

edit oops - didn't see the "++". Still, it's an alternative.

Append an int to char*

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