How do I use an if statment to check if a string literal is a certain word or phrase

I am still new to coding and was playing around trying to figure out simple conversations and i get the following errors while compiling:

error: could not convert 'str.std::basic_string<_CharT, _Traits, _Alloc>::operator=, std::allocator >(((const char*)"good"))' from 'std::basic_string' to 'bool' if (str = "good") {

and

error: could not convert 'str.std::basic_string<_CharT, _Traits, _Alloc>::operator=, std::allocator >(((const char*)"bad"))' from 'std::basic_string' to 'bool' else if (str = "bad") {

I get those errors from the Following code; Bare in mind I'm still very new to this:

// random practice on conversation
#include <iostream>
#include <string>
using namespace std;

int main ()
{
    string str;
    string bad;

    cout << "How has your day been?  \n";
    cin >> str;
    if (str = "good") {
        cout << "Thats good to hear!\n";
    }
    else if (str = "bad") {
        cout << "That's too bad, what happened?  \n";
        cin >> bad;
        cout << "I'm sorry to hear that...\n";
    }
    else {
        cout << "I'm sorry, I couldn't understand you...\n";
    }
}