How to clear a priority_queue that uses user-defined compare?
From std::priority_queue documentation, I reduced the use of priority_queue to the case I need (= queue with user-defined compare)
>> cat test.cpp
#include <functional>
#include <queue>
#include <vector>
#include <iostream>
#include <utility>
auto queue_cmp = [](std::pair<int, double> const& lhs,
std::pair<int, double> const& rhs) {
return lhs.second > rhs.second; // Custom order.
};
typedef std::priority_queue<std::pair<int, double>,
std::vector<std::pair<int, double>>,
decltype(queue_cmp)> custom_queue;
template<typename T>
void print_queue(T q) { // NB: pass by value so the print uses a copy
int s = 0;
while(!q.empty()) {
std::pair<int, double> elem = q.top();
std::cout << s << ": " << elem.first << ", " << elem.second << std::endl;
q.pop();
s ;
}
std::cout << '\n';
}
int main() {
custom_queue q(queue_cmp);
for (int n = 10; n < 20; n ) {
double val = static_cast <double>(rand())/(static_cast<double>(RAND_MAX));
q.push(std::pair<int, double>(n, val));
}
print_queue(q);
//q = custom_queue(queue_cmp);
}
This runs OK
>> g -o test test.cpp
>> ./test
0: 15, 0.197551
1: 18, 0.277775
2: 16, 0.335223
3: 11, 0.394383
4: 19, 0.55397
5: 17, 0.76823
6: 12, 0.783099
7: 13, 0.79844
8: 10, 0.840188
9: 14, 0.911647
Now I need to reset q so, following priority queue clear method, I uncomment the last line in test.cpp... And get this compilation error as if I get this correctly, copy constructors are deleted on lamdba:
>> g -o test test.cpp
test.cpp: In function ‘int main()’:
test.cpp:34:31: error: use of deleted function ‘std::priority_queue<std::pair<int, double>, std::vector<std::pair<int, double> >, <lambda(const std::pair<int, double>&, const std::pair<int, double>&)> >& std::priority_queue<std::pair<int, double>, std::vector<std::pair<int, double> >, <lambda(const std::pair<int, double>&, const std::pair<int, double>&)> >::operator=(std::priority_queue<std::pair<int, double>, std::vector<std::pair<int, double> >, <lambda(const std::pair<int, double>&, const std::pair<int, double>&)> >&&)’
34 | q = custom_queue(queue_cmp);
| ^
In file included from /usr/include/c /12/queue:64,
from test.cpp:2:
/usr/include/c /12/bits/stl_queue.h:498:11: note: ‘std::priority_queue<std::pair<int, double>, std::vector<std::pair<int, double> >, <lambda(const std::pair<int, double>&, const std::pair<int, double>&)> >& std::priority_queue<std::pair<int, double>, std::vector<std::pair<int, double> >, <lambda(const std::pair<int, double>&, const std::pair<int, double>&)> >::operator=(std::priority_queue<std::pair<int, double>, std::vector<std::pair<int, double> >, <lambda(const std::pair<int, double>&, const std::pair<int, double>&)> >&&)’ is implicitly deleted because the default definition would be ill-formed:
498 | class priority_queue
| ^~~~~~~~~~~~~~
/usr/include/c /12/bits/stl_queue.h:498:11: error: use of deleted function ‘<lambda(const std::pair<int, double>&, const std::pair<int, double>&)>&<lambda(const std::pair<int, double>&, const std::pair<int, double>&)>::operator=(const<lambda(const std::pair<int, double>&, const std::pair<int, double>&)>&)’
test.cpp:7:19: note: a lambda closure type has a deleted copy assignment operator
7 | auto queue_cmp = [](std::pair<int, double> const& lhs,
| ^
Is there a way to reset q in this case?
EDIT
bool queue_cmp(std::pair<int, double> const& lhs,
std::pair<int, double> const& rhs) {
return lhs.second > rhs.second; // Custom order.
};
Doesn't help
CodePudding user response：
Not in C 17.
C 20 gives capture-less lambdas a default constructor. But in earlier language versions, the constructor is deleted.
Really, you should not be using a lambda. Just use a named struct.
CodePudding user response：
As noted above in C 17 capture-less lambdas do not have default constructor.
Since a lambda is actually an anonymous class with a operator() method, you can use a named struct for the same purpose.
Your comparator will become:
struct queue_cmp
{
bool operator()(std::pair<int, double> const& lhs, std::pair<int, double> const& rhs) const
{
return lhs.second > rhs.second; // Custom order.
};
};
The rest of the code is almost the same.
You need to drop the decltype because queue_cmp is already a type, and there's no need to supply an instance of the comparator when constructing a queue:
typedef std::priority_queue<std::pair<int, double>,
std::vector<std::pair<int, double>>,
queue_cmp> custom_queue;
template<typename T>
void print_queue(T q) { // NB: pass by value so the print uses a copy
int s = 0;
while (!q.empty()) {
std::pair<int, double> elem = q.top();
std::cout << s << ": " << elem.first << ", " << elem.second << std::endl;
q.pop();
s ;
}
std::cout << '\n';
}
int main() {
custom_queue q;
for (int n = 10; n < 20; n ) {
double val = static_cast <double>(rand()) / (static_cast<double>(RAND_MAX));
q.push(std::pair<int, double>(n, val));
}
print_queue(q);
q = custom_queue{};
}