Why does std::string not provide a conversion to const char*?

Question

This is more of a policy or a historical question. Why was it decided not to provide a const char * conversion for std::string? Were there a fear someone might do printf("%s", s) and believe it would automatically convert? Are there any open discussions on this issue?

Answer 1

Automatic casts are almost always evil. If there were a cast to const char *, a std::string could be also automatically cast to other pointer types and that could lead to hard to find bugs. There is the c_str() method that returns const char * so you can still achieve what you need. Also, the typecast is not logically correct - std::string is not equivalent to const char *.

Answer 2

The string class internally need not store the string with a terminating 0. In fact it doesn't even have to store them in contiguous memory if it didn't want to. Therefore an implicit cast doesn't make sense, since it may be a costly operation.

The c_str() function then gives you the c-string. Depending on how the library stores it internally this function may have to create a temporary. This temporary is only valid until you modify the string.

It is unfortunately however since a string could just been specified to be a c-string internally. This wouldn't lead to any loss of functionality and would allow an implicit conversion.

Edit The standard does basically imply the memory is contiguous (if accessed through data() or the [] operator), though it need not be internally, and certainly not null terminated. Likely all implementations store the 0 as well. If this were standardized then the implicit conversion could be safely defined.