Best way to format string in C++

Question

In javascript I can format a string using template string

const cnt = 12;
console.log(`Total count: ${cnt}`);

if I work with python I can use the f-string:

age = 4 * 10
f'My age is {age}'

But, if I working with C++(17) what is the best solution to do this (if it is possible)?

Install [libfmt](https://github.com/fmtlib/fmt), then you can do `fmt::print("Total count: {}", cnt);`. — HolyBlackCat, Oct 08 '20 at 20:26
`std::cout << "Total count: " << cnt << std::endl;` Works with stringstream too — Vlad Feinstein, Oct 08 '20 at 20:26
I don't know what's the problem well. but if u want to use numbers with the strings here is an example. `size_t age = 40; std::string str = "My age is "+ std::to_string(age);` — asmmo, Oct 08 '20 at 20:27

selbie · Answer 1 · 2021-03-31T20:27:33.807

13

I think the simpler way is std::to_string:

std::string str = "My age is ";
str += std::to_string(age);

std::ostringstream also works nicely and can be useful as well:

With this at the top of your source file

#include <sstream>

Then in code, you can do this:

std::ostringstream ss;
ss << "My age is " << age;
std::string str = ss.str();

edited Mar 31 '21 at 20:27

answered Oct 08 '20 at 20:31

selbie

score 10 · Answer 2 · answered Oct 08 '20 at 20:29

10

you can use sprintf

sprintf(dest_string, "My age is %d", age).

but using sprintf will rase an error, so best use snprintf:

snprintf(dest_string, size , "My age is %d", age);

where size is the is the maximum number of bytes.

answered Oct 08 '20 at 20:29

David Frucht

11

Welcome to StackOverflow David. Unfortunately, answers in "C" for questions tagged as C++, while valid, tend to get downvoted. (Disclaimer: I didn't downvote, but have been a victim of this culture as well) – selbie Oct 08 '20 at 20:35
8

While this solution *can* work in C++, it is based on C and is not a good solution for C++, as it lacks adequate type checking and buffer overflow checking, things that C++ has solved with other solutions. – Remy Lebeau Oct 08 '20 at 20:37
Which other solution? Interesting that *A Tour of C++ (Third edition)* of Stroustrup (so a very short and essential guide of C++ to C++23) explains also such C method (but none of the rest of c library). Until std::format is in wide use (still not available on many compilers now), there is no perfect C++ solution. I think we should trust Stroustrup. – Giacomo Catenazzi Jun 28 '23 at 14:25

score 0 · Answer 3 · edited Oct 08 '20 at 20:35

0

If you want to write to stdout:

const int cnt = 12;
std::cout << "Total count: " << cnt << "\n";

Writing to any other stream works similar. For example replace std::cout by a std::ofstream to write to a file. If you need the formatted string:

std::ostringstream oss;
oss << "Total count: " << cnt << "\n";
std::string s = oss.str();
std::cout << s;

edited Oct 08 '20 at 20:35

Remy Lebeau

answered Oct 08 '20 at 20:33

3 Answers3