qsort of a structure in c++ - c++

Question

I try to sort a structure but I have this error :
error: cannot convert ‘std::vector<Node>’ to ‘void*’ for argument ‘1’ to ‘void qsort(void*, size_t, size_t, __compar_fn_t)’
qsort(nodes,nodes.size(), sizeof(Node), dataClustering::compare);
This is my code :
compare function :
int compare(const void * node1, const void * node2){
string name1 = ((const struct Node*)node1)->name;
string name2 = ((const struct Node*)node2)->name;
int start1 = ((const struct Node*)node1)->start;
int start2 = ((const struct Node*)node2)->start;
if(name1 <= name2 && start1 <= start2){
return -1;
}
else if(name1 > name2 && start1 > start2){
return 1;
}
else{
return 0;
}
}
Call of compare in another function :
qsort(allNodes,allNodes.size(), sizeof(Node), compare);
And the header in .hpp file (I try to put it in static but this don't solve the error):
int compare(struct Node *, struct Node *);

Answer 1

You can pass vector to C-style sort, for example, like this:
qsort(&allNodes[0], allNodes.size(), sizeof(Node), compare);
You would be better off using std::sort, which works with vectors natively:
bool compareNodes(const Node& lhs, const Node& rhs) {
return (lhs.name < rhs.name)
|| (lhs.name == rhs.name && lhs.start < rhs.start);
}
...
std::sort(allNodes.begin(), allNodes.end(), compareNodes);
in C++11/C++14 you can define comparison as a lambda, too:
std::sort(allNodes.begin(), allNodes.end(), [] (const auto& lhs, const auto& rhs) {
return (lhs.name < rhs.name)
|| (lhs.name == rhs.name && lhs.start < rhs.start);
});
(the code above uses auto for lambda arguments, a C++14 feature).

Answer 2

The error is that you
cannot convert ‘std::vector<Node>’ to ‘void*’
and thus, a std::vector<Node> cannot possibly be a suitable first argument to a function like qsort with signature
void (void*, size_t, size_t, __compar_fn_t)
While there are things you could do to use qsort here, you really shouldn't: you should use std::sort from the C++ standard library:
using std::sort;
sort(begin(allNodes), end(allNodes), compare_function);
Here, compare_function should be a function that acts like < and takes two arguments of the appropriate type (e.g. the actual type of the objects you're sorting or const references to them, not pointers to them or void pointers) and returns bool if the first argument is less than the second. (compare_function doesn't actually have to be a function: it can also be any object that has an appropriate operator() or a lambda)

Answer 3

If you want to just sort, use
#include <algorithm>
...
bool myfunction (const Node &lhs, const Node &lhs) {
if(lhs.name < rhs.name) return true;
return (lhs.name == rhs.name && lhs.start < rhs.start));
}
...
std::sort(allNodes.begin(), allNodes.end(), compare_func);
Further read: std::sort
If you want to use cstyle qsort(which I suggest you should not), add a wrapper function to do the conversion of void * to Node * and change the first argument of qsort to static_cast<void *>(&allNodes[0])