C++ Extract std::string from a variable-length argument list

Question

Hey everyone! I'm trying to make a simple copy of sprintf that returns the formatted string, but I am coming into a small snag...

Apparently, using a variable-length argument list you cannot pass a std::string instance.

I already have the parser working properly with int, double, float, char, const char*, char*... I have yet to get strings to work. :\

In case you're wondering, this is the compile error I get: /root/learncpp/StringFormat/main.cpp:8: warning: cannot pass objects of non-POD type 'struct std::string' through '...'; call will abort at runtime

The main reason I'm doing this is so that I can have convenient formatting without having to rely on 3rd party libraries, yet still not have to append ".c_str()" to every string instance I use.

Help with this would be appreciated. Perhaps there's a different version of variable-length argument lists made specifically for C++?

EDIT: I have just realized, if you pass a pointer to a string (i.e. using the & prefix) it works well. All you have to do is dereference a string pointer in the custom sprintf, while passing the address of the std::string!

Still, it would be nice to see if there's any way to support string directly through variable-length argument lists. Thanks!

Jerry Coffin · Accepted Answer · 2011-04-17 01:47:17Z

3

No -- as the compiler said, you're only allowed to pass objects of POD type to a variadic function.

What you normally want to do is eliminate using a variadic function in the first place, such as by using an iostream instead of something like printf (or a stringstream instead of sprintf).

answered Apr 17, 2011 at 1:47

Jerry Coffin

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13 Comments

FurryHead Over a year ago

Well, the reason I like sprintf is the way you can automatically convert ints/doubles into a string, and the nice format of using it. iostreams/stringstreams just seem clumsy to me when it's mixing many variables and spacers, i.e. "Hello %s, the time is %d:%d:%d, on the %dth day of the %dth month."

Jerry Coffin Over a year ago

Have you looked at Boost format? boost.org/doc/libs/1_46_0/libs/format/index.html

James McNellis Over a year ago

@FurryHead: The simplest way to use a stringstream for trivial conversions is to use boost::lexical_cast; it is straightforward to implement and you can find the basic implementation in Herb Sutter's article, "The String Formatters of Manor Farm"

johnsyweb Over a year ago

@FurryHead: I would say the streams version is easier to read and far, far easier to maintain as everything is inline.

Matthieu M. Over a year ago

@Johnsyweb: and then you get a French (me) who asks you to translates the text, and suddenly IOStreams are no longer convenient, at all.

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