Hello!
When I compile the following code, I get this error message "error:
passing ... discards qualifiers" and I don't understand why.
Could anybody help me?
Thank you!
John
-----main.cpp------
....
map<Pair<string, string>, int> myMap;
myMap(Pair<string, string>("bla", "blablabla")) = 2;
....
-----Pair.h-------
template <class T1, class T2>
class Pair
{
public:
T1 first;
T2 second;
Pair();
Pair(T1 first, T2 second);
virtual ~Pair();
bool operator == (Pair<T1, T2> p);
bool operator < (Pair<T1, T2> p);
};
-----Pair.cpp-------
#include "Pair.h"
template <class T1, class T2>
Pair<T1, T2>::Pair()
{
}
template <class T1, class T2>
Pair<T1, T2>::Pair(T1 first, T2 second)
{
this->first = first;
this->second = second;
}
template <class T1, class T2>
Pair<T1, T2>::~Pair()
{
}
template <class T1, class T2>
bool Pair<T1, T2>::operator == (Pair<T1, T2> p)
{
return (first == p.first && second == p.second) || (first == p.second
&& second == p.first);
}
template <class T1, class T2>
bool Pair<T1, T2>::operator < (Pair<T1, T2> p)
{
return first < p.first || (!(p.first < first) && second < p.second);
} 2 19077
John wrote: Hello!
When I compile the following code, I get this error message "error: passing ... discards qualifiers" and I don't understand why.
Could anybody help me?
See comments inline.
Thank you!
John
-----main.cpp------ ... map<Pair<string, string>, int> myMap; myMap(Pair<string, string>("bla", "blablabla")) = 2;
use qualified names, std::map, std::string. ...
-----Pair.h------- template <class T1, class T2> class Pair { public: T1 first; T2 second; Pair(); Pair(T1 first, T2 second); virtual ~Pair(); bool operator == (Pair<T1, T2> p); bool operator < (Pair<T1, T2> p);
These should both be (const Pair& p) const.
};
-----Pair.cpp------- #include "Pair.h"
template <class T1, class T2> Pair<T1, T2>::Pair() { }
template <class T1, class T2> Pair<T1, T2>::Pair(T1 first, T2 second) { this->first = first; this->second = second;
Prefer initialiser lists.
}
template <class T1, class T2> Pair<T1, T2>::~Pair() { }
template <class T1, class T2> bool Pair<T1, T2>::operator == (Pair<T1, T2> p) { return (first == p.first && second == p.second) || (first == p.second && second == p.first); }
template <class T1, class T2> bool Pair<T1, T2>::operator < (Pair<T1, T2> p) { return first < p.first || (!(p.first < first) && second < p.second); }
--
Ian Collins.
"John" <cy***@purecode.ch> wrote in message
news:11************@sicinfo3.epfl.ch Hello!
When I compile the following code, I get this error message "error: passing ... discards qualifiers" and I don't understand why.
Could anybody help me?
Thank you!
John
-----main.cpp------ ... map<Pair<string, string>, int> myMap; myMap(Pair<string, string>("bla", "blablabla")) = 2;
The standard library includes a templated functional called less. less is a
struct that has an operator() that takes two arguments and applies < to
them. The two arguments are declared as const, which means that it is not
permissible to call any operator on them which is not likewise const.
Calling a non-const operator "discards qualifiers", i.e., discards const.
By default, map uses less to compare keys. less is calling your operator<.
Since your operator< is non-const, you are getting the error message. -----Pair.h------- template <class T1, class T2> class Pair { public: T1 first; T2 second; Pair(); Pair(T1 first, T2 second); virtual ~Pair(); bool operator == (Pair<T1, T2> p);
bool operator == (const Pair<T1, T2> p) const;
(the first const says you aren't going to modify the "other" Pair; the
second const says the Pair isn't going to modify itself.) You need to make
the same changes to the implementation --- see below
bool operator < (Pair<T1, T2> p);
bool operator < (const Pair<T1, T2> p) const;
};
Unless your compiler supports export, this separation into .h and .cpp files
won't work. You need to put the implementation into the .h file when using
templates (more precisely, the implementation needs to be visible in the
translation unit that is using it --- the compiler has to be able to see it;
you can't rely on the linker).
-----Pair.cpp------- #include "Pair.h"
template <class T1, class T2> Pair<T1, T2>::Pair() { }
template <class T1, class T2> Pair<T1, T2>::Pair(T1 first, T2 second) { this->first = first; this->second = second; }
template <class T1, class T2> Pair<T1, T2>::~Pair() { }
template <class T1, class T2> bool Pair<T1, T2>::operator == (Pair<T1, T2> p)
bool Pair<T1, T2>::operator == (const Pair<T1, T2> p) const
{ return (first == p.first && second == p.second) || (first == p.second && second == p.first); }
template <class T1, class T2> bool Pair<T1, T2>::operator < (Pair<T1, T2> p)
bool Pair<T1, T2>::operator < (const Pair<T1, T2> p) const
{ return first < p.first || (!(p.first < first) && second < p.second); }
--
John Carson This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
by: DanielESFA |
last post by:
Hey guys :)
This is a bit of a funny one... We're four guys working on the same project,
everybody using KDevelop and g++ on Linux. Three of us are using Mandrake,
with g++ 3.4.3 and 3.4.1....
|
by: Jason |
last post by:
I have a function (Inet_ntop) that returns const char * and if I try to
assign that return value to a char * variable, I get the gcc error message:
warning: assignment discards qualifiers from...
|
by: Charlie Zender |
last post by:
Hi,
I am unable to compile a large body of code with extremely pedantic
compile time checks activate, so that warnings cause errors.
With GCC 3.3.1, I do this with
gcc -std=c99 -pedantic...
|
by: Markus Dehmann |
last post by:
I am trying to make a pair with a string and an auto_ptr:
#include <iostream>
#include <map>
using namespace std;
int main(){
auto_ptr<intp(new int(3));
make_pair("x",p);
}
|
by: Lycan. Mao.. |
last post by:
Hello, I'm trying to write a function adapter object, but it fails
with the above information. Can you help me.
template <typename _Predicate>
struct Unary_negate {
typedef typename...
| |
by: Pietro Cerutti |
last post by:
i Group,
to my understanding, defining a function parameter as "const" means that
the function is not going to change it.
Why does the compiler says "return discards qualifiers from pointer...
|
by: Thelma Lubkin |
last post by:
I use my own matrix and vector classes. I wrote them before such
things were generally available and I've stuck with them ever since.
I've just added an Octonion class derived from the vectors...
|
by: Andre |
last post by:
Hi All,
When I compile the following piece of code with gcc, I get 3 "warning:
initialization discards qualifiers from pointer target type" messages
which refer to the 3 lines marked in the...
|
by: Ruben |
last post by:
error: passing `const Weight' as `this' argument of `float
Weight::wgt()' discards qualifiers
seems to be some sort of standard error format that I'm not understanding.
I have code that...
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However,...
|
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers,...
| |
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows...
|
by: tracyyun |
last post by:
Dear forum friends,
With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each...
|
by: agi2029 |
last post by:
Let's talk about the concept of autonomous AI software engineers and no-code agents. These AIs are designed to manage the entire lifecycle of a software development project—planning, coding, testing,...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The...
|
by: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated ...
| |
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence...
| |