no known conversion for templated vs const non-templated vector

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.