Accessing private values in cpp using pointers

Question

For some reason the getter methods doesn't work. They are public, so I have no idea what's wrong.

for (std::vector<Document>:: const_iterator it = v.begin(); it != v.end(); ++it)
{
    cout << it->getName() << endl;
    counter += it->getLength();
}

error: passing 'const Document' as 'this' argument of 'void Document::getName()' discards qualifiers [-fpermissive] cout << it->getName() << endl;

error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream}' and 'void') cout << it->getName() << endl;

error: passing 'const Document' as 'this' argument of 'void Document::getLength()' discards qualifiers [-fpermissive] counter += it->getLength();

error: invalid operands of types 'int' and 'void' to binary 'operator+' counter += it->getLength();

Hmm, is there a way to can we do (int) (it->getLength()) for the last problem

and can we do for the other one:

std::ostringstream value;   
value << (*it).getName();
cout << getName << endl;

They're not const, so you can't call them with a const_iterator. It has nothing to do with accessibility and everything to do with const-correctness. And you can't print the result of something returning void. — Qaz
– Qaz, Commented Feb 21, 2014 at 23:49
ah ok, but the other two problems still remains. +1 for the rapid remark. — OnTheFly
– OnTheFly, Commented Feb 21, 2014 at 23:54
One of those addresses two errors and if you want to be pedantic, I could also say you can't add the result of something returning void to something else. I fail to see the other problem. — Qaz
– Qaz, Commented Feb 21, 2014 at 23:56

Martin Drozdik · Accepted Answer · 2014-02-22 01:33:50Z

3

Just declare the getters to be const:

class Document
{
public:
    std::string getName() const;
    int getLenght() const;
};

and specify their return values.

The error messages are not very readable, but in gcc:

error: passing A as B argument of C discards qualifiers

is almost always caused by trying to modify something that is const.

The other messages are clear however:

error: no match for 'operator<<'
(operand types are 'std::ostream {aka std::basic_ostream}' and 'void')
cout << it->getName() << endl;

that is you are trying to pass a std::ostream and void to an operator.

edited Feb 22, 2014 at 1:33

answered Feb 21, 2014 at 23:55

Martin Drozdik

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1 Comment

OnTheFly Over a year ago

have any idea why both are void?

Jerry Coffin · Accepted Answer · 2014-02-21 23:57:25Z

2

Although you don't show the relevant code, the error messages show enough to take a pretty good guess at the problem.

Your class apparently looks something like this:

class Document { 
// ...
public:
    void getName() { /* ... */ }   
    void getLength() { /* ... */ }
    // ...
};

To fix the problem, you need to change getName and getLength to 1) return values, and 2) be const member functions, something on this general order:

class Document { 
// ...
public:
    std::string getName() const { /* ... */ }   
    size_t getLength() const { /* ... */ }
    // ...
};

answered Feb 21, 2014 at 23:57

Jerry Coffin

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