c++string字符串的比较是否相等问题

#include <iostream>
#include <string>
using namespace std;
int main()
{
    if("abc"=="abc")
    {
        cout<<"abc等于abc"<<endl;
    }
    else
    {
        cout<<"abc不等于abc"<<endl;
    }
    if("abc"=="ab")
    {
        cout<<"abc等于ab"<<endl;
    }
    else
    {
        cout<<"abc不等于ab"<<endl;
    }
    string str1="abc",str2="abc",str3="ab";
    if(str1==str2)
    {
        cout<<str1<<"等于"<<str2<<endl;
    }
    else
    {
        cout<<str1<<"不等于"<<str2<<endl;
    }
    if(str1==str3)
    {
        cout<<str1<<"等于"<<str3<<endl;
    }
    else
    {
        cout<<str1<<"不等于"<<str3<<endl;
    }
    return 0;
}

#include <iostream>
#include <string>
#include <string.h>
using namespace std;
int main()
{
    string str1 = "abc", str2 = "abc";
    if ( strcmp( str1.c_str(), str2.c_str() ) == 0 )
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
    return 0;
}

#include <string>
#include <string.h>
#include <iostream>
using namespace std;
string t1 = "helloWorld";
string t2 = "helloWorld";
int main(){
    if (t1 == t2)
    {
        cout<<"***t1 ,t2 是一样的\n";
        cout<<"这是正确的\n";
    }
    // error
    if (t1.c_str() == t2.c_str())
    {
        cout<<"@@@t1 ,t2 是一样的\n";
    }
    // error
    if (t1.c_str() == "helloWorld")
    {
        cout<<"===t1 ,t2 是一样的\n";
    }
    if (strcmp(t1.c_str(),t2.c_str()) == 0)
    {
        cout<<"###t1 ,t2 是一样的\n";
        cout<<"这是正确的\n";
    }
    return 0;
}