no known conversion for templated vs const non-templated vector - c++

Question

In my actual code, I included a library, and as soon as I did that, it started crashing. I managed to sort of extract some of that code into this minimal example, that demonstrates the same kind of error:
// g++ -std=c++11 -g -o test-classcall.exe test-classcall.cpp
#include <iostream>
#include <vector>
#include <stdio.h>
class Cat
{
public:
int Age;
Cat() : Age(0) {}
};
std::vector<Cat> myPCats;
typedef std::vector<Cat> TDVectCats;
TDVectCats myTDCats;
void loopSomeCats() {
printf("this function just to cause searching for matching calls\n");
}
void loopSomeCats(TDVectCats& incats) {
std::vector<Cat>::iterator iter;
for(iter = incats.begin(); iter != incats.end(); iter++) {
printf("hm\n");
}
}
const std::vector<Cat> & getSomeCats() {
return myPCats;
}
void doSomething() {
loopSomeCats(getSomeCats());
}
int main() {
myTDCats.push_back(Cat());
myTDCats.push_back(Cat());
myPCats.push_back(Cat());
doSomething();
std::cout << "Hello World! " << std::endl;
return 0;
}
The result is:
$ g++ -std=c++11 -g -o test-classcall.exe test-classcall.cpp
test-classcall.cpp: In function ‘void doSomething()’:
test-classcall.cpp:36:29: error: no matching function for call to ‘loopSomeCats(const std::vector<Cat>&)’
loopSomeCats(getSomeCats());
^
test-classcall.cpp:36:29: note: candidates are:
test-classcall.cpp:20:6: note: void loopSomeCats()
void loopSomeCats() {
^
test-classcall.cpp:20:6: note: candidate expects 0 arguments, 1 provided
test-classcall.cpp:24:6: note: void loopSomeCats(TDVectCats&)
void loopSomeCats(TDVectCats& incats) {
^
test-classcall.cpp:24:6: note: no known conversion for argument 1 from ‘const std::vector<Cat>’ to ‘TDVectCats& {aka std::vector<Cat>&}’
What especially confuses me, is the last "no known conversion for argument 1 from ‘const std::vector<Cat>’ to ‘TDVectCats& {aka std::vector<Cat>&}’", as if it cannot convert a vector of something, into the vector of the same something, just because of typedef? Or it maybe has to do with the const - but I simply cannot see what I need to change, in order to have a call like loopSomeCats(getSomeCats()); succeed...

Answer 1

You can't pass a reference to a const object to a non-const reference.
loopSomeCats takes a std::vector<Cat>& as argument, and you want to pass a const std::vector<Cat>& to it, but that's not possible.
The const would mean that you don't want anyone to modify the return value, but if you pass it to a function which just takes a non-const reference, then theoretically the function can modify the reference, and you don't want that.
You should drop the const if you want the return value to be modified.