error: Iso c++ forbids comparison between pointer and integer [c++], splitting sentence into words

I am trying to split the sentence into words by checking the characters from the string one by one.

#include <iostream>

using namespace std;

int main()
{
    string a;
    getline(cin,a);
    string word = "";
    for(int i=0;i=a.size();i++)
    {
        if(a[i]==" ") //error iso here
        {
            cout<<word<<endl;
            word="";
        }
        else
        {
            word+=a[i];
        }
        cout<<word<<endl;
    }

}