Re: Const/non-const pointer returning method
* Jens Thoms Toerring, on 25.05.2010 20:03:
I am rather new to C++ and have run into a problem where I
haven't found an answer yet by searching (probably didn't find
the right search terms). I Have this simple program:
#include<iostream>
class A
{
public:
A( int i ) : m_ip( new int[ i ] ) { }
int const * ip( ) const { std::cout<< "const\n"; return m_ip; }
int * ip( ) const { std::cerr<< "non-const\n"; return m_ip; }
private:
int * m_ip;
};
int main( )
{
A a( 10 );
int const * ip = a.ip( );
std::cout<< ip[ 2 ]<< '\n';
}
Do you?
<example>
C:\test> gnuc x.cpp
x.cpp:8: error: 'int* A::ip() const' cannot be overloaded
x.cpp:7: error: with 'const int* A::ip() const'
C:\test> msvc x.cpp
x.cpp
x.cpp(8) : error C2556: 'int *A::ip(void) const' : overloaded function differs
only by return type from 'const int *A::i
p(void) const'
x.cpp(7) : see declaration of 'A::ip'
x.cpp(8) : error C2373: 'A::ip' : redefinition; different type modifiers
x.cpp(7) : see declaration of 'A::ip'
x.cpp(8) : error C2143: syntax error : missing ';' before '<<'
x.cpp(8) : warning C4517: access-declarations are deprecated; member
using-declarations provide a better alternative
x.cpp(8) : error C2886: 'std::cerr' : symbol cannot be used in a member
using-declaration
C:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\iostream(14) : see declaration of 'std::cerr'
x.cpp(8) : error C2238: unexpected token(s) preceding ';'
x.cpp(8) : error C2059: syntax error : 'return'
x.cpp(8) : error C2238: unexpected token(s) preceding ';'
x.cpp(10) : error C2143: syntax error : missing ';' before ':'
x.cpp(10) : error C2059: syntax error : ':'
x.cpp(12) : error C2059: syntax error : '}'
x.cpp(12) : error C2143: syntax error : missing ';' before '}'
x.cpp(12) : error C2059: syntax error : '}'
x.cpp(17) : error C2264: 'A::ip' : error in function definition or declaration;
function not called
C:\test> _
</example>
My exectation was that when calling ip() to get a const pointer
the compiler would be able to figure out I want it to use the
first ip() method that returns a const pointer. But it turns out
that always the second one is invoked.
I rather doubt it.
First your program needs to compile.
I'm not sure why and if
there's a way that I can make it pick the second one (short of
using different names for the methods)? I also noticed the same
effect when using const versus non-const references as return
values, also there the non-const returning function is called
eben when one asks for a const reference. Does all this only
work when overloading the [] operator?
As a first step, get your program to compile.
You can safely disregard the earlier comments in this thread until your code
compiles.
And then it may turn out that the code that compiles is not what those comments
are about.
Cheers & hth.,
- Alf
--
blog at <url: http://alfps.wordpress.com>