Is it Safe to Use `std::string::c_str()` with Temporary Strings in C ?

Potential Pitfalls of Using std::string::c_str() with Temporary Strings

Consider the following C code snippet:

void consumer(char const* p) { std::printf("%s", p); }

std::string random_string_generator();

consumer(random_string_generator().c_str());

This code raises the question of whether it is well-formed according to the C standard.

The concern stems from the fact that random_string_generator() returns a temporary std::string object. We then take the c_str() pointer, potentially leaving the temporary std::string object vulnerable to destruction before the pointer is used.

To understand the implications, we need to delve into the lifetime of temporaries in C .

According to the C standard, the lifetime of a temporary extends until the end of the full expression in which it was created. In this case, the full expression is the call to consumer. Therefore, the temporary std::string object will not be destroyed until after the consumer function has finished executing.

Thus, the c_str() pointer we obtained will remain valid throughout the call to consumer, making our code well-formed and safe to use.

It is important to note that this guarantee only holds for the lifetime of the full expression. If the c_str() pointer is stored and used outside of this context, the behavior becomes undefined as the temporary std::string object may have already been destroyed.

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