c++ const error

Question

 bool check_integrity( int pos ) const { if (( pos <= 0 ) || ( pos > max_seq ) || ( pos >= _length + _beg_pos )) { cerr << "!! invalid position: " << pos << " Cannot honor request\n"; return false; } if ( _isa == ns_unset ) { cerr << "!! object is not set to a sequence." << " Please set_sequence() and try again!\n"; return false; } if ( pos > _elem->size()){ cout << "check_integrity: calculating " << pos - _elem->size() << " additional elements\n"; ( this->*_pmf )( pos ); } return true; } public: typedef void (num_sequence::*PtrType)( int ); private: PtrType _pmf;

The above code clip is part of class "num_sequence". I got an error for the following line:

( this->*_pmf )( pos );

The error is: 'const num_sequence *const this' Error: the object has type qualifiers that are not compatible with the member function

Thanks!

OJ. · Accepted Answer · 2012-08-10 05:19:19Z

check_integrity is a const function so functions it calls must also be const, hence to call a PtrType function is must also be const.

Try this:

typedef void (num_sequence::*PtrType)( int ) const;

NB: I didn't compile this :) Just thinking out loud.

AnT stands with Russia · Accepted Answer · 2012-08-10 06:56:06Z

You are trying to call a non-const member function pointed by _pmf for a constant object *this. This is a violation of const-correctness rules.

Either declare your PtrType as

typedef void (num_sequence::*PtrType)( int ) const;

or remove const from your check_integrity function

bool check_integrity( int pos ) { ...

Either this or that. You didn't provide enough information for someone else to decide which is the right thing to do in this case.

mmirzadeh · Accepted Answer · 2012-08-10 05:21:53Z

You need to change

typedef void (num_sequence::*PtrType)( int );

to

typedef void (num_sequence::*PtrType)( int ) const;

since you are calling the function from a const function

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