Is there a way to implement sprintf's buffer allocation automatically? [duplicate]

Question

I want to use mysprintf () instead of sprintf () for automatic buffer size allocation.

Is there any problem with mysprintf()? Or can you recommend a better way?

char s[256]; sprintf(s, "%s-%s-%s", "abcdefg", "abcdefg", "abcdefg");

string s = mysprintf("%s-%s-%s", "abcdefg", "abcdefg", "abcdefg"); string mysprintf(const char* format, ...) { int ret; char* buf; va_list ap; va_start(ap, format); ret = vasprintf(&buf, format, ap); va_end(ap); if (ret == -1) { return {}; } string out(buf); free(buf); return out; }

You are walking a path of great pain, trying to take all of the disadvantages of sprintf with little to show for it. Has anyone introduced you to parameter packs? This will still be painful as all hell, but type safe. — user4581301
– user4581301, Commented Sep 4, 2019 at 4:52
Seems fine to me in principle (ignoring type safety), although strictly speaking it's not exception-safe; if std::string's constructor throws, you've leaked memory. — jamesdlin
– jamesdlin, Commented Sep 4, 2019 at 5:25
Unless you want to learn about what's wrong with your existing code. — user202729
– user202729, Commented Sep 4, 2019 at 5:26
At the very least, you should wrap the allocated buf in a std::unique_ptr with a custom deleter to call free() — Remy Lebeau
– Remy Lebeau, Commented Sep 4, 2019 at 6:12
vasprintf() is GNU specific. If your compiler is compatible with C99 - or supports the C99 standard library - you can use vsnprintf() to, (1) obtain a required buffer (2) allocate a container (std::string or a std::vector<char>) of appropriate size and (3) write the data directly to that container. The benefit of this alternative is that all the allocations are cleanly deallocated (no need to call free() manually, and no opportunity to inadvertently not do so). — Peter
– Peter, Commented Sep 4, 2019 at 6:28

lilington · Accepted Answer · 2019-09-04 05:05:47Z

-1

You don't need to write your own function, use asprintf() instead.

But notice that it is a USE_GNU stuff.

Remember to free()

answered Sep 4, 2019 at 5:05

lilington

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2 Comments

jamesdlin Over a year ago

The point is to make a C++ wrapper around asprintf so that memory is managed via RAII.

Benjamin Over a year ago

Yes, @ jamedin's comment is my point.

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