Reglas de sintaxis typedef de C++

Question

Reglas de sintaxis typedef de C++

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DavidS · Answer 1

For the life of me I cannot find a good explanation of what are the rules that are used to convert a typedef to a C++ statement. The simple cases I understand. But consider this from Danny Kalev:

typedef char * pstr;
int mystrcmp(const pstr, const pstr); //wrong!

Danny Kalev then writes:

The sequence const pstr actually means char * const (a const pointer to char); not const char * (a pointer to const char.

I cannot find anywhere the rule to explain why "const pstr" would be converted to "char * const".

Gracias por cualquier ayuda.

preguntado el 22 de mayo de 12 a las 17:05

DavidS

This seems logical, no? You are defining pstr type as pointer to char. So const pstr is a const pointer to char, char * const. - arrowd

Simple rule of thumb. typedef != #define. If you want what you are describing, you must use a macro. - Richard J. Ross III

If you use the post-const form consistently, it makes a lot more sense. pstr const == char * const. The pre-const form is an exception and accordingly it can make things confusing. - Benjamin Lindley

Paul Manta · Answer 2

Eso es porque pstr es un alias para char* y cuando lo hagas const pstr es un poco como decir const (char*) y no (const char)*.

contestado el 22 de mayo de 12 a las 17:05

Paul Manta

Thanks. This sheds some light on things. I still would be interested in knowing if there was documentation that shows that "const pstr" gets expanded to "const (char *)" and not "const char *". - Davids

Rob Kennedy · Answer 3

A typedef isn't like a macro; it doesn't just perform simple text replacement. The typedef defines a single unit, and the additional const applies to the entire thing. The unit defined is a pointer, so applying const to it give you a const pointer.

The outcome you expected would require the const to "reach inside" the pstr type to apply to something internal. It would get worse the more pointer levels were declared inside that type. Consider typedef char*** pppstr. To make that a char const***, la const would have to be inserted three levels deep dentro de pppstr type. It's better for the rule to consistently apply const to the outer level, no matter how complicated the type definition really is.