Is it safe to pass a temporary `std::string::c_str()` to a function in C ?

Examining the Durability of Temporary Strings in std::string::c_str()

In the realm of C programming, the utilization of temporary strings often raises concerns about their memory management and durability. This article delves into the specifics of using std::string::c_str() with temporary strings and examines its conformance to the C standard.

Problem:

Consider the following code snippet:

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

std::string random_string_generator();

consumer(random_string_generator().c_str());

Does the code exhibit correct behavior, or does it have potential pitfalls related to the lifespan of temporaries?

Answer:

According to the C standard, after creating a temporary std::string object and obtaining its c_str() pointer, the string object's lifetime extends until:

• A non-const method of the string object is invoked
• The string object is explicitly destructed

In the provided code example, the temporary std::string object (random_string_generator()) is destructed at the end of the full expression, not before or after. Since consumer is invoked within the same expression, the code is considered well-formed.

However, it's crucial to note that if consumer were to store the pointer for future use, the code would no longer be safe. This is because the lifetime of the temporary string object would have ended before consumer could access the pointer.

Conclusion:

The code in question is indeed well-formed according to the C standard, as long as it is used within the same expression. Understanding the lifespan rules for temporaries is essential to avoid memory management issues and ensure correct program behavior.

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