Calling string.find() function and assigning its return value to variable inside if condition

I have trouble understanding what's exactly happening when i put .find() function inside if condition. Similar call in for loop seems to work good.

Part of my code goes like this:

    size_t endPos = strbuf.find(endSeq)+endSeq.size();
    cout<<endPos<<endl;

    if (size_t endPosif = strbuf.find(endSeq)+endSeq.size() != string::npos)
    {
        cout<<endPosif<<endl;
    }

    for (size_t endPosfor = strbuf.find(endSeq)+endSeq.size();endPosfor != string::npos; 
        endPosfor=strbuf.find(startSeq,endPosfor+1)+endSeq.size())
    {
        cout<<endPosfor<<endl;
    }

Output:

466
1
466

What causes this behavior? I would like to use endPosif inside if statement.