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c++string-formattingstringstreamprintf
AShelly
AShelly

Reputation: 35580

Is default stringstream formatting the same as snprintf's?

I'm looking to replace some snprintf calls with stringstreams. I need to ensure the output is identical.

given long l = some_input_var;

do 

std::ostringstream str;
str << l;
return str.str();

and 

buf str[24];
snprintf(str, 24, "%ld", l);
return std::string(str);

return identical strings?

What about for int and "%d" or double and "%f" ?

Upvotes: 2

Views: 796

Answers (3)

James Kanze
James Kanze

Reputation: 153977

In the standard, the C++ formatting is defined in terms of printf format specifiers, so you can be pretty sure that simple formatters will work the same. There are, however, things the iostream doesn't support (and vice versa): there's no iostream equivalent of "% d", for example, and the precision specification is ignored when outputting a string (probably because iostream doesn't know whether the string would be part of the format string, or correspond to a "%s").

Also, don't forget that all of the format options except width are sticky; you should save them, and restore them when you've formatted the element which needs them.

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477338

Yes. But it's a long story: iostreams formatting works via the stream's codec's formatting facet. The details are fairly involved; see C++11 27.7. The default facet's conversion uses sprintf for input formatting and the strtoull-type functions for output; see 22.4.2.1 for details.

Also and similarly, the new <string> conversion functions std::to_string use the default sprintf format, and the various std::stoul-like functions use the C library's strtoul-like functions.

Upvotes: 2

nhed
nhed

Reputation: 5999

Let say that for that case the answer is yes, I wouldn't trust it to be the same for all cases, especially with the pains of making all the alignments right.

Did you consider Boost::format?

Something like return boost::str(boost::format("%ld") % l); should work and will provide for an easier migration path. Though differences do exist.

Upvotes: 0

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