Is constructing an object in an argument list and passing a pointer to internal data of the object to the function safe?

Question

Is the C++ code below well-formed? Will the std::string get destroyed before or after the function finishes executing?

void my_function(const char*);

...

my_function(std::string("Something").c_str());

I know I could do my_function("Something"), but I am using std::string this way to illustrate my point.

Lombroso

But seriously, the temporary string will die at the end of the expression. That'll be after the function call. So long as my_function doesn't store that pointer somewhere with a longer life you're good. – Drucill

Some nearly-canonical documentation. – Drucill

I'm surprised I can't find a good duplicate of this. Closest I got is this. – Purplish

Here's the canonical documentation and some supporting information on when the expression ends. Took longer to find than I thought it would. But go with the stuff at CPP reference. I reads like it was translated from Martian, but in a way that's because it was. – Drucill

It's hard to fathom why you propose that construction over simply my_function("Something");. I mean, sure, std::string is widely favored over C strings in C++, but when you have a function that wants a const C string anyway, how does it make sense to use a std::string to make a temporary copy of a perfectly good C string to pass to it? – Coil

@JohnBollinger It's just an example of a more general case. See the last sentence in the question. – Lizbeth

@Purplish Plenty of good duplicates to choose from: google.com/search?q=c_str+as+temporary+site:stackoverflow.com – Upandcoming

Answer 1

Will the std::string get destroyed before or after the function finishes executing?

Temporary objects are destroyed (with some exceptions, none relevant here) at the end of the full-expression in which they were materialized (e.g. typically the end of an expression statement).

The std::string object here is such a temporary materialized in the full-expression that spans the whole my_function(std::string("Something").c_str()); expression statement. So it is destroyed after my_function returns in your example.

Crossbill