C++ mastermind erreur (code joint)

if(a=solution1)

if(a==solution1)

#include <vector>
#include <cassert>
#include <iostream>

typedef std::vector<unsigned int> solution_t;

unsigned int randint(unsigned int max){
  //...
}

void generer_code(
  solution_t & solution,
  unsigned int nb_couleur,
  unsigned int nb_cases
){
  solution = solution_t(nb_cases);
  for(unsigned int i=0;i<nb_cases;++i){
    solution[i] = randint(nb_couleur);
  }
}

void show_solution(const solution_t & sol){
    for(unsigned int i=0;i<sol.size();++i){
      std::cout << sol[i] << ' ';
    }
    std::cout << std::endl;
}

void read_proposition(
  solution_t & prop
){
  //Lire sur std::cin la proposition
}

unsigned int get_nb_valid(
  const solution_t & solution,
  const solution_t & proposition
){
  unsigned int nb_valid=0,i;
  assert(solution.size()==proposition.size())
  for(std::size_t i=0;i<solution.size();++i){
    if (solution[i]==proposition[i]){
      std::cout << i << "eme nombre correct" << std::endl;
      ++nb_valid;
    }
  }
  return nb_valid;
}

unsigned int get_nb_mal_place(
  const solution_t & solution,
  const solution_t & proposition
){
  unsigned int nb_mal_place=0,i;
  assert(solution.size()==proposition.size())
  for(std::size_t i=0;i<solution.size();++i){
    if (solution[i]!=proposition[i]){
      for(std::size_t j=0;j<solution.size();++j){
        if (i!=j && solution[i]==proposition[j]){
          ++nb_mal_place;
          break;
        }
      }
    }
  }
  return nb_mal_place;
}

int main(){

  solution_t sol;
  solution_t prop;
  generer_code(sol,8,4);
  unsigned int nb_essai=5,nb_valid,nb_mal_place;
  bool gagne = false;

  for(unsigned int i=1;i<=nb_essai && !gagne;++i){
    std::cout << "Essai (" << i << '/' <<nb_essai << ')' << std::endl;
    //Lire le code proposé et le mettre dans prop
    //....
    //Comparer la proposition à la solution
    nb_valid = get_nb_valid(sol,prop);
    if (nb_valid == sol.size()){
      gagne = true;
    }
    std::cout << nb_valid << " nombres sont valides" << std::endl;
    nb_mal_place = get_nb_mal_place(sol,prop);
  }

  if (gagne) std::cout << "Gagné !" << std::endl;
  else{
    std::cout << "Perdu :( La solution était " << std::endl;
    show_solution(sol);
  }
  return 0;
}