C++ - Errors when trying to insert class into map - c++

Question

I have a map like so map<string, unique_ptr<Base>> variables and I am trying to insert data into the map variables.insert(make_pair("foo", new Int(10))) but I am getting to following errors:
error: no matching function for call to ‘std::map<std::__cxx11::basic_string<char>, std::unique_ptr<Base>>::insert(std::pair<const char*, Int*>)’
variables.insert(make_pair("test", new Int(10)));
error: no type named ‘type’ in ‘struct std::enable_if<false, void>’
template<typename _Pair, typename = typename
This is my code:
class Base {
public:
Base() {};
virtual ~Base() {};
};
class Int : public Base {
public:
Int(int i) {
this->i = i;
}
Int operator=(int i) {
this->i = i;
}
int i;
};
void set() {
map<string, unique_ptr<Base>> variables;
variables.insert(make_pair("test", new Int(10)));
}
I think I need a fresh pair of eyes to look at this I'm not sure what this issue is, thanks!
Edit
I'm trying to make a heterogeneous map and there's a class for each data type. But I still get the same error no matter how many there are.

Answer 1

Note: This answer only applies to older versions of the main three compilers:
GCC: Applies to 5.3.1 or earlier. May apply to any version earlier than 6.1.0, but I haven't tested this.
Clang: Applies to 3.7.1 or earlier. May apply to any version earlier than 3.8.0, but I haven't tested this.
Visual Studio: Applies to 19.00.23506.0 or earlier. May apply to any version earlier than 19.00.23720.0, but I haven't tested this.
Conversely, if you have GCC 6.1.0 or later, Clang 3.8.0 or later, or Visual Studio 19.00.23720.0 or later, the original code should compile as is, without either of the modifications suggested in this answer.
[Thanks goes to AndyG for pointing out that it works with later versions of GCC & Clang.]
The problem appears to be that it isn't creating your unique_ptr from your raw pointer.
If you can use C++14, try std::make_unique().
void set() {
map<string, unique_ptr<Base>> variables;
variables.insert(make_pair("test", make_unique<Int>(10)));
}
If you can't, then try something like this:
void set() {
map<string, unique_ptr<Base>> variables;
variables.insert(make_pair("test", unique_ptr<Int>(new Int(10))));
}
Interestingly, I noticed a slight difference in how different compilers handle this. Using the following slightly modified version of your code as a test program:
#include <map>
#include <memory>
#include <iostream>
class Base {
public:
Base() {};
virtual ~Base() {};
};
class Int : public Base {
public:
Int(int i) {
this->i = i;
}
Int& operator=(int i) {
this->i = i;
// You forgot to return something.
return *this;
}
int i;
};
void set() {
using namespace std;
map<string, unique_ptr<Base>> variables;
variables.insert(make_pair("test", new Int(10)));
// C++14:
// variables.insert(make_pair("test", make_unique<Int>(10)));
// C++11:
// variables.insert(make_pair("test", unique_ptr<Int>(new Int(10))));
// Cheap hack for testing.
cout << static_cast<Int*>(variables["test"].get())->i << endl;
}
int main() {
set();
}
Most compilers* will fail to compile this, unless the initial line is commented out and either of the fixes is uncommented. However, the online MSVC compiler seemed to be able to compile it fine, without needing to uncomment either of the lines. Curiously, the version of MSVC available on Rextester failed to compile it without uncommenting one of the two lines.
* Tested online, with TutorialsPoint GCC, MSVC 2015 online, and Rextester Clang, GCC, and MSVC.