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StringPrintf: A Typesafe printf Family for C++


August, 2005: StringPrintf: A Typesafe printf Family for C++

Unnamed Temporary C++ Objects

The main technique that makes StringPrintf possible is the use of unnamed temporary objects. Most C++ programmers use this technique very often, although most of the time, it is not explicit. For example, having a set of std::strings, the set function insert only accepts references to objects of type std::string. Nevertheless, you can correctly write:

<b>std::set<std::string> stringSet;</b>
<b>stringSet.insert("ABC");</b>

This is possible because the compiler, when finding the C-string "ABC", looks for a way to convert the C-string to a const reference to a std::string object. It then finds that basic_string (the template base class of std::string) has a constructor basic_string(const char *s). The compiler uses this constructor to create an unnamed temporary object of std::string out of "ABC", and passes it to std::set.insert(). The line

<b>stringSet.insert("ABC");</b>

is expanded to

<b>stringSet.insert(std::string("ABC"));</b>

A unnamed temporary std::string object is created and its reference is passed to the insert function. When the insert function returns, the temporary object is destroyed.


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