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POC++: how to get fprintf results as a std::string w/o sprintf
 

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  1. POC++: how to get fprintf results as a std::string w/o sprintf
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    <p>I am working with an open-source UNIX tool that is implemented in C++, and I need to change some code to get it to do what I want. I would like to make the smallest possible change in hopes of getting my patch accepted upstream. Solutions that are implementable in standard C++ and do not create more external dependencies are preferred.</p> <p>Here is my problem. I have a C++ class -- let's call it "A" -- that currently uses fprintf() to print its heavily formatted data structures to a file pointer. In its print function, it also recursively calls the identically defined print functions of several member classes ("B" is an example). There is another class C that has a member std::string "foo" that needs to be set to the print() results of an instance of A. Think of it as a to_str() member function for A.</p> <p>In pseudocode:</p> <pre><code>class A { public: ... void print(FILE* f); B b; ... }; ... void A::print(FILE *f) { std::string s = "stuff"; fprintf(f, "some %s", s); b.print(f); } class C { ... std::string foo; bool set_foo(std::str); ... } ... A a = new A(); C c = new C(); ... // wish i knew how to write A's to_str() c.set_foo(a.to_str()); </code></pre> <p>I should mention that C is fairly stable, but A and B (and the rest of A's dependents) are in a state of flux, so the less code changes necessary the better. The current print(FILE* F) interface also needs to be preserved. I have considered several approaches to implementing A::to_str(), each with advantages and disadvantages:</p> <ol> <li><p>Change the calls to fprintf() to sprintf()</p> <ul> <li>I wouldn't have to rewrite any format strings</li> <li>print() could be reimplemented as: fprint(f, this.to_str());</li> <li>But I would need to manually allocate char[]s, merge a lot of c strings , and finally convert the character array to a std::string</li> </ul></li> <li><p>Try to catch the results of a.print() in a string stream</p> <ul> <li>I would have to convert all of the format strings to &lt;&lt; output format. There are hundreds of fprintf()s to convert :-{</li> <li>print() would have to be rewritten because there is no standard way that I know of to create an output stream from a UNIX file handle (though <a href="http://synflood.at/blog/index.php?/archives/456-One-word-of-warning-about-stdio_filebuf.html" rel="nofollow noreferrer">this guy says it may be possible</a>).</li> </ul></li> <li><p>Use Boost's string <a href="http://www.boost.org/doc/libs/1_36_0/libs/format/doc/format.html" rel="nofollow noreferrer">format library</a></p> <ul> <li>More external dependencies. Yuck.</li> <li>Format's syntax is different enough from printf() to be annoying: </li> </ul> <p>printf(format_str, args) -> cout &lt;&lt; boost::format(format_str) % arg1 % arg2 % etc</p></li> <li><p>Use Qt's <a href="https://doc.qt.io/qt-5/qstring.html#asprintf%3E" rel="nofollow noreferrer">QString::asprintf()</a></p> <ul> <li>A different external dependency.</li> </ul></li> </ol> <p>So, have I exhausted all possible options? If so, which do you think is my best bet? If not, what have I overlooked?</p> <p>Thanks.</p>
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