Is it safe to use c_str() as a parameter in std::exception?

Question

Is it safe to pass c_str() as a parameter when constructing std::exception? Please let me know if handling exceptions like this is a bad idea. In my project all error messages are returned from a function as std::string and then thrown as a std::exception.

#include <iostream>

int main()
{
    try {
        std::string message="! Something went wrong.";
        throw std::exception(message.c_str());
    }
    catch (std::exception ex) {
        std::cerr << ex.what() << std::endl;
    }
}

No, it's not, and I'm pretty sure this has been answered before. — πάντα ῥεῖ, Sep 22 '22 at 15:12
In your question, you may want to explain in a few words why you are concerned that this may be a bad idea. — Andreas Wenzel, Sep 22 '22 at 15:13
Setting aside the dangling pointer, `std::exception` doesn't have a constructor that takes an argument, except for its copy constructor. — François Andrieux, Sep 22 '22 at 15:13

Remy Lebeau · Accepted Answer · 2022-09-22T15:39:18.840

3

std::exception does not have a contructor that takes a const char* or a std::string.

std::runtime_error (and its descendants) does have constructors for both. And yes, it is perfectly safe (well, provided that memory is not low) to pass the message.c_str() pointer to this constructor. std::runtime_error will copy the character data into its own internal memory, allowing message to be destroyed after the exception is thrown.

If you want to throw std::exception itself with a string message, you will have to derive a custom class from it and implement your own string buffer for what() to return a pointer to. In which case, you have to be careful not to return an invalid const char* pointer from what(). std::runtime_error handles that for you, so you should derive from that instead.

edited Sep 22 '22 at 15:39

answered Sep 22 '22 at 15:13

Remy Lebeau

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...and because it has a constructor taking `const char*` passing `message.c_str()` is wrong. – 463035818_is_not_an_ai Sep 22 '22 at 15:14
perfectly safe? `message` will be destroyed and the pointer returned from `c_str` will be dangling – 463035818_is_not_an_ai Sep 22 '22 at 15:15
@463035818_is_not_a_number you are wrong on both counts. `std::runtime_error` does not store the passed `char*` pointer as-is. It makes a copy of the data. – Remy Lebeau Sep 22 '22 at 15:19
hm yes, would be strange if the constructor would not copy the string ;) – 463035818_is_not_an_ai Sep 22 '22 at 15:24

Is it safe to use c_str() as a parameter in std::exception?

1 Answers1