C++ equivalent of sprintf?

Question

I know that std::cout is the C++ equivalent of printf.

What is the C++ equivalent of sprintf?

score 75 · Accepted Answer · edited Dec 22 '22 at 03:00

75

std::ostringstream

Example:

#include <iostream>
#include <sstream> // for ostringstream
#include <string>

int main()
{
  std::string name = "nemo";
  int age = 1000;
  std::ostringstream out;
  out << "name: " << name << ", age: " << age;
  std::cout << out.str() << '\n';
  return 0;
}

Output:

name: nemo, age: 1000

edited Dec 22 '22 at 03:00

Benjamin Loison

3,782
4
16
33

answered Feb 13 '11 at 07:51

Vijay Mathew

26,737
4
62
93

4

I don't think that sprintf writes to stdout. I would remove the insertion statement above. – Raffi Khatchadourian May 18 '11 at 01:08
105

How is this even remotely similar to `sprintf (...)`? You cannot format the data arbitrarily, you have to rely on the type being known when you feed it into the stream using the `<<` operator. – Andon M. Coleman Feb 27 '14 at 15:40
1

I need to agree with @AndonM.Coleman about this one. This is not really a sprintf replacement. [This](http://qt-project.org/doc/qt-5/qstring.html#arg) would be more like that, but this is Qt. – László Papp Mar 06 '14 at 17:25
1

as @vinkris says in his answer, iomanip achieves formatting. Instead of printing to stdoit, I would say "result = out.str()". – Dmitri Oct 03 '14 at 04:09
sprintf / snprintf allows formating and printing to a user allocated character array, may be on stack. In case of snprintf(), it ensures there is no overrun. Here we are allocating memory multiple times and the caller has no direct access it. Have to convert to a string to get the output. An std::ostream with a custom std::streambuf, which takes user buffer would be a better match - of course construction/destruction of ostream/streambuf adds more in-efficiency. – MGH Aug 09 '18 at 20:39
@AndonM.Coleman: You are right, but the context of the answer makes sense when considering the OP's initial sentence. – jxh Feb 19 '19 at 23:55
@MGH: `<<` supports formatting (e.g. `<< std::hex() << 42`), otherwise these are all features, not bugs. I tend to think that in most `sprintf()` uses, the requirement to manage memory and protect from over-run is a hassle that should be avoided if possible. C++ offers you easier coding practices (optimize for development time) and are usually not that worse off in terms of CPU time. If you need something super-optimized, there are a few options (including writing C-style or even `asm`) otherwise - come to the darkside of idiomatic C++, we have cookies. – Guss Nov 25 '20 at 21:05

janm · Answer 2 · 2019-08-27T14:59:02.587

41

Update, August 2019:

It looks like C++20 will have std::format. The reference implementation is {fmt}. If you are looking for a printf() alternative now, this will become the new "standard" approach and is worth considering.

Original:

Use Boost.Format. It has printf-like syntax, type safety, std::string results, and lots of other nifty stuff. You won't go back.

edited Aug 27 '19 at 14:59

answered Feb 13 '11 at 08:53

janm

17,976
1
43
61

20

... unless you are concerned about the size of your executable.. :P – pradyunsg Sep 07 '14 at 15:11
How much of an impact would this have? The Boost dependency would be header-only, no linking, correct? – Ken Williams Nov 09 '17 at 19:25
1

@KenWilliams Yes, Boost.Format is header only. A simple "hello, world" test on my Mac increases from 10kB to 78kB. In a real project the extra size will be amortised across compilation units (give the right linker options), and the type safety brings other benefits. – janm Nov 09 '17 at 21:14
When executable size bloats, always try to use shared libraries! – Hunter Kohler Feb 11 '22 at 16:18

score 21 · Answer 3 · edited Dec 01 '11 at 19:58

21

sprintf works just fine in C++.

edited Dec 01 '11 at 19:58

Chris Klepeis

9,783
16
83
149

answered Feb 13 '11 at 08:27

Steve Rowe

19,411
9
51
82

7

I think the OP meant STL rather than C++. – Jean-Michaël Celerier Aug 01 '13 at 08:26
5

sprintf requires you to allocate the character buffer. I would like something like the "append" method of "std::string" that allows me to add formatted data, and taking care of allocating behind the scenes. – Victor Eijkhout Mar 04 '15 at 18:52

Erik Aronesty · Answer 4 · 2022-06-15T15:35:53.923

12

Here's a nice function for a c++ sprintf. Streams can get ugly if you use them too heavily.

std::string string_format(std::string fmt, ...) {
    int size=100;
    std::string str;
    va_list ap;

    while (1) {
        str.resize(size);
        va_start(ap, fmt);
        int n = vsnprintf(&str[0], size, fmt.c_str(), ap);
        va_end(ap);
   
        if (n > -1 && n < size) {
            str.resize(n); // Make sure there are no trailing zero char
            return str;
        }
        if (n > -1)
            size = n + 1;
        else
            size *= 2;
    }
}

In C++11 and later, std::string is guaranteed to use contiguous storage that ends with '\0', so it is legal to cast it to char * using &str[0].

edited Jun 15 '22 at 15:35

answered Dec 02 '11 at 21:11

Erik Aronesty

11,620
5
64
44

Very nice solution! I've adapted it here: http://stackoverflow.com/a/3742999/15161 to more closely fit `sprintf`-usage. – slashmais Dec 27 '12 at 18:07
12

Illegal, though: `(char*) str.c_str()` casts away `const`. – MSalters Mar 18 '13 at 12:36
also there's buffer overflow problem lurking – Barney Szabolcs Nov 04 '15 at 12:56
@MSalters Correct. There's a legal way to do that in C++11. – Catherine Dec 28 '15 at 11:25
@whitequark: Feel free to add that as an answer. On Stack Overflow, good question remain open to allow new answers. – MSalters Dec 28 '15 at 12:50
Not illegal, no casting problem, and no possible buffer overflow. Please read the specs. – Erik Aronesty Jun 03 '16 at 15:17
Fantastic! You declared `n` twice though – theicfire Oct 24 '19 at 21:19
There is a interesting bug when you try to append something to the result of this function. Took me a while to figure out it was appended at position 100+ – uncletall Oct 12 '20 at 09:05
I got `warning: passing an object of reference type to 'va_start' has undefined behavior [-Wvarargs]` on Clang++-10 – Sapphire_Brick Oct 20 '20 at 15:38
@Sapphire_Brick just change the arg to pass fmt by value. it won't actually copy the string on modern compilers anyway – Erik Aronesty Oct 27 '20 at 20:40

score 7 · Answer 5 · edited May 23 '17 at 12:34

7

You can use iomanip header file to format the output stream. Check this!

edited May 23 '17 at 12:34

Community

1
1

answered Aug 04 '14 at 14:11

vinkris

139
1
6

Why did somebody downvote this? Isn't iomanip the pure-C++ way of achieving formatting in streams? I think the goal here is to avoid storing data in C-style strings, which is achieved with iomanip. – Dmitri Oct 03 '14 at 04:06

score 1 · Answer 6 · answered Jul 31 '19 at 13:49

Depending on what exactly you plan on sprintf()ing, std::to_string() might be useful and more idiomatic than other options:

void say(const std::string& message) {
 // ...
}

int main() {
  say(std::to_string(5));
  say("Which is to say " + std::to_string(5) + " words");
}

The main advantage of std::to_string(), IMHO, is that it can be extended easily to support additional types that sprintf() can't even dream of stringifying - kind of like Java's Object.toString() method.

score -1 · Answer 7 · edited May 25 '17 at 01:30

-1

Use a stringstream to achieve the same effect. Also, you can include <cstdio> and still use snprintf.

edited May 25 '17 at 01:30

bstpierre

30,042
15
70
103

answered Feb 13 '11 at 07:52

regality

6,496
6
29
26

C++ equivalent of sprintf?

7 Answers7

Linked

Related