Inserting in map<string, STRUCT> error - c++

Question

I have a map with the struct defined as under:
struct kv_string {
std::string value;
long long exp_time;
kv_string(const std::string& v): value(v), exp_time(-1) {}
};
Now when I'm trying to insert a new structure using
else if(qargs[0] == "set"){
if(qargs.size()==3){
kv_map.insert(std::make_pair( qargs[1], kv_string(qargs[2])));
}
}
(qargs is a vector<string>), I get the following error:
> In file included from /usr/include/c++/4.8/map:61:0,
> from structures.h:5:
> /usr/include/c++/4.8/bits/stl_map.h: In instantiation of ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key,
> _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::basic_string<char>; _Tp = kv_string; _Compare =
> std::less<std::basic_string<char> >; _Alloc =
> std::allocator<std::pair<const std::basic_string<char>, kv_string> >;
> std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = kv_string;
> std::map<_Key, _Tp, _Compare, _Alloc>::key_type =
> std::basic_string<char>]’:
> /usr/include/c++/4.8/stdexcept:281:48: required from here
> /usr/include/c++/4.8/bits/stl_map.h:469:59: error: no matching function for call to ‘kv_string::kv_string()’
> __i = insert(__i, value_type(__k, mapped_type()));
> ^
> /usr/include/c++/4.8/bits/stl_map.h:469:59: note: candidates are:
> structures.h:11:9: note: kv_string::kv_string(const string&)
> kv_string(const std::string& v): value(v), exp_time(-1) {}
> ^
> structures.h:11:9: note: candidate expects 1 argument, 0 provided
> structures.h:8:8: note: kv_string::kv_string(const kv_string&)
> struct kv_string {
> ^
> structures.h:8:8: note: candidate expects 1 argument, 0 provided
> make: *** [server_main.o] Error 1
I have also tried adding an additional constructor kv_string(){}, but it gives a segmentation fault.

Answer 1

You want this:
kv_map.insert(std::make_pair(qargs[1], kv_string(qargs[2]));
Or this:
kv_map.emplace(qargs[1], kv_string(qargs[2]);
Or, in C++17:
kv_map.try_emplace(qargs[1], qargs[2]);
The []-operator default-initializes a new element (if one doesn't exist for the given key), but your type kv_string is not default-constructible. So you cannot use that operator. The above operations are more powerful than the []-operator, too: they return an iterator to the element at the key, and information about whether the key already existed.

Answer 2

The C++ compiler emitted this error message complaining about the lack of a default constructor for your kv_string class:
/usr/include/c++/4.8/bits/stl_map.h:469:59: error: no matching function for call to ‘kv_string::kv_string()’
__i = insert(__i, value_type(__k, mapped_type()));
If you read the documentation for std::map::operator[], you'll see that the mapped type (in your case kv_string) must be default-constructible.
So, if it makes sense for your own design, you could just add a default constructor to your kv_string struct:
struct kv_string {
// ...
// Default constructor
kv_string() : exp_time(-1 /* or whatever default value */) {}
};
As a side note, I would also mark your kv_string(const std::string&) constructor as explicit, to avoid implicit conversions from strings.