One of my C++ questions that has been solved!

View this post at the Programmers Heaven C++ forum

Posted on 7 May 2009 at 3:58 AM

Dear fellow PH-users,

When using std::seach_n in a const-correct way I ran into problems. I found a workaround, but I hope one of you can give an in-depth explanation.

The following code should compile, but it doesn't (under the IDE C++ Builder 6.0 Enterprise edition with the Borland compiler BCB.exe version 6.0.10.157):


const std::string s = "abc***def";

const int n = 3; //Number of repeats

std::search_n( s.begin(),s.end(),n,'*');

The compile-error given is 'Cannot modify const object', because 'n' is of type 'const int'. Although the real problem is in the std::search_n algorithm, I use the following workaround:


const std::string s = "abc***def";

const int n = 3; //Number of repeats

std::search_n( s.begin(),s.end(),static_cast<int>(n),'*');

Due to the static_cast to integer type, this compiles nicely. But what I also tried is to const_cast the const integer n to integer type, as below:


const std::string s = "abc***def";

const int n = 3; //Number of repeats

std::search_n( s.begin(),s.end(),const_cast<int>(n),'*');

This again gives the compile-error 'Cannot modify const object'.

IMHO the const_cast is better then the static_cast (because I want the constness away).

So my question is: why does static_cast work and why doesn't const_cast work?

Thanks in advance, Bilderbikkel

Answer by Sergey (thanks!)

As section 5.2.11 Const cast [expr.const.cast] of ISO C++ standard suggests, there are limited amount of conversion that can be performed using const_cast, so const_cast operator can be briefly described as applicable to two pointers to the same type with various cv-qualifiers. So point is that you can't const_cast to any type other that pointer or reference.

And static_cast worked becouse it created new temporary object from existing one, it is was similar to use "const int n = 10; search_n( s.begin(), s.end(), int( n), '*');". But I suppose that these casts are redundant becouse arguments are passed by value so algorithm don't affects "outer" variables.

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) const_cast does not work to remove constness in argument

Answer by Sergey (thanks!)