Question

I need to read input from user. The input value may be string type or int type.
If the value is int then the program insert the value into my object.
Else if the value is string then it should check the value of that string, if it’s “end” then the program ends.

Halda h; //my object
string t;
int tint;
bool end=false;

while(end!=true)
{
    if(scanf("%d",&tint)==1)
    {
        h.insert(tint);
    }
    else if(scanf("%s",t)==1)
    {
      if(t=="end")
          end=true;
      else if(t=="next")
          if(h.empty()==false) 
              printf("%d\n",h.pop());
          else 
              printf("-1\n");
    }
}

The problem is that scanning string doesn’t seem to work properly.
I’ve tried to change it to: if(cin>>t) and it worked well.
I need to get it work with scanf.

Answer 1

The specifier %s in the scanf() format expects a char*, not a std::string.

From C11 Standard (C++ Standard refers to it about the C standard library):

Except in the case of a % specifier, the input item (or, in the case of a %n directive, the
count of input characters) is converted to a type appropriate to the conversion specifier. If
the input item is not a matching sequence, the execution of the directive fails: this
condition is a matching failure. Unless assignment suppression was indicated by a *, the
result of the conversion is placed in the object pointed to by the first argument following
the format argument that has not already received a conversion result. If this object
does not have an appropriate type, or if the result of the conversion cannot be represented
in the object, the behavior is undefined.

Anyway, here there’s is no real reason to prefer the C way, use C++ facilities. And when you use the C library, use safe functions that only reads characters up to a given limit (just like fgets, or scanf with a width specifier), otherwise you could have overflow, that leads again to undefined behavior, and some errors if you’re luck.