Why is the `c_str()` pointer of a temporary string object valid in `bar()` after its creating function `foo()` has returned?

The Lifetime of Objects in foo() and bar()

In C , objects created during function calls are considered temporary objects. Understanding their lifetime is crucial for ensuring correct program behavior.

Consider the following code:

std::string foo() {
  std::string out = something...;
  return out;
}

void bar( const char* ccp ) {
  // do something with the string..
}

bar( foo().c_str() );

Question:

Why is the c_str() pointer of the temporary object returned by foo() valid in the bar() function even after foo() has completed?

Answer:

According to the C standard, a temporary object is destroyed once the full expression that contains its creation is completed. In this case, the full expression is:

bar( foo().c_str() );

In ASCII art, the lifetime of the temporary object looks like this:

____________________   full-expression ranges from 'b' to last ')'
bar( foo().c_str() );
     ^^^^^          ^
       |            |
     birth       funeral

The temporary object is created when foo() is called (birth) and destroyed after the expression finishes executing (funeral), ensuring that the c_str() pointer remains valid throughout the execution of bar().

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