Inserting objects into vector using lower_bound with compare struct - c++

Question

I have this class:
class Mail {
public:
Mail(const string & msg) : msg(msg) {}
const string msg;
};
And this structure, which compares two Mail objects:
struct Compare {
bool operator()(const Mail & mail, Mail const & mail2) const {
return mail.msg < mail2.msg;
}
};
I want to have a vector with Mail objects sorted by their message const string msg. However, when I try to insert new object into vector using lower_bound, I get many errors, including:
passing ‘const string as ‘this’ argument discards qualifiers.
int main() {
vector <Mail> mails;
Mail mail2("1");
mails.push_back(mail2);
const string msg = "2";
Mail mail(msg);
auto low = lower_bound(mails.begin(), mails.end(), mail, Compare());
// mails.push_back(mail); // OK
mails.insert(low, mail); // passing ‘const string as ‘this’ argument discards qualifiers
return 0;
}
I dont yet understand const usage much and can't figure out, which const is wrong.
I am sorry, if this has already been asked, but I haven't found an answer to this problem yet.

Answer 1

The errors in C++ can sometimes be hard to diagnose. My tip is to always start at the top and resolve that one first. In this case, there's a long list of them, but they're all really about the same thing -- the assignment operator for Mail cannot be generated.
Think of it this way, the compiler is being helpful and is trying to generate (and inside lower_bound(), use) this function:
Mail& operator=( const& Mail mail )
{
msg = mail.msg;
return *this;
}
But it can't because that assignment in the body is invalid due to msg being const. You can't really write it yourself either since you also can't assign to a const variable.
Usually you don't need member variables to be const because they become const if the instance of the class is itself const:
const auto mail1 = Mail{"1"};
auto mail2 = Mail{"2"};
mail1.msg = "3"; // FAIL! msg is const since mail1 is const
mail2.msg = "4"; // Ok! msg is not const
If you do need a const member, you can't use assignment operators with the class. Them's the breaks.
Remove that const and all works:
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
class Mail {
public:
Mail(const string & msg) : msg(msg) {}
string msg; //////////////////////////////// Not const!
};
struct Compare {
bool operator()(const Mail & mail, Mail const & mail2) const {
return mail.msg < mail2.msg;
}
};
int main() {
vector <Mail> mails;
Mail mail2("1");
mails.push_back(mail2);
const string msg = "2";
Mail mail(msg);
auto low = lower_bound(mails.begin(), mails.end(), mail, Compare());
// mails.push_back(mail); // OK
mails.insert(low, mail); // OK!
return 0;
}
See it run live on Coliru.
Footnotes:
You could use a lambda for the comaparator to avoid some boilerplate around the class Compare:
const auto low = lower_bound( begin(mails), end(mails), mail,
[]( const auto& mail1, const auto& mail2 )
{ return mail1.msg < mail2.msg; } );
You can use vector::emplace_back() to construct items in place, avoiding a copy. The following blocks do the same thing in effect, but the second is more efficient:
const auto mail = Mail{"2"};
mails.push_back( mail2 ); // Copies
mails.emplace_back("2"); // Creates it right in the vector
Consider using vector::reserve() if you know how many items you'll put in your vector.

Answer 2

The problems here pertain to the deleted copy assignment operator and the deleted move assignment operator because of the const string msg; member in the Mail class:
Deleted implicitly-declared copy assignment operator
A defaulted copy assignment operator for class T is defined as deleted if any of the following is true:
T has a non-static data member of non-class type (or array thereof) that is const;
Deleted implicitly-declared move assignment operator
The implicitly-declared or defaulted move assignment operator for class T is defined as deleted if any of the following is true:
T has a non-static data member that is const;