type cast? template.... What is this cpp line of code?

Question

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behaviour of const_cast

I need to understand what this line means

char A = strdup(const_cast<char*>(aString.c_str()));

I understand what strdup does from this:

strdup() - what does it do in C?

strdup expects a const char pointer. Its the <,> part of the above line that confuses me.

`const_cast` is unnecessarily casting the result of `c_str()` from a `char const*` to a `char*` -- it's pointless here. — ildjarn, Feb 21 '12 at 19:01
That line doesn't mean anything; it won't compile. (Something about initializing a `char` with a pointer type.) — Rob Kennedy, Feb 21 '12 at 19:33

score 3 · Accepted Answer · edited Feb 21 '12 at 19:08

3

const_cast< type > is a C++ operator. You can read about it here.

I don't understand why it's needed here, since (assuming aString is of type std::string) c_str() already returns the const char* which strdup requires, and in any case adding constness is done implicitly.

Only if the function receives a non-const parameter it's required, and even then it's usually not recommended.

edited Feb 21 '12 at 19:08

Lightness Races in Orbit

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answered Feb 21 '12 at 19:05

Asaf

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score 1 · Answer 2 · answered Feb 21 '12 at 19:09

The cast operator,

char * p = const_cast<char*>(q);

allows you to remove the constness (provided that q is a const char *).

In general const_cast<T> can be used to add or remove the const qualifier.

However, as strdup should be taking a const char * the cast here is not needed.

Robᵩ · Answer 3 · 2012-02-21T19:42:06.827

                         aString

A variable, probably of type std::string

                         aString.c_str()

A const char * which points to nul-terminated array of chars

       const_cast<char*>(aString.c_str())

A cast, converting the value that is inside () to the type named inside <>. In this case, converting from const char* to char*. This is a C++-style cast (see also static_cast<>(), dynamic_cast<>(), and reinterpret_cast<>()).

strdup(const_cast<char*>(aString.c_str()));

As you say, you know what strdup does. Since the signature of strdup is probably char* strdup(const char*), it turns out that this particular cast is pointless.

type cast? template.... What is this cpp line of code?

3 Answers3