C++ PPP Chapter 8 Exercise 1 - c++

Question

Modify the calculator program from Chapter 7 to make the input stream an explicit parameter (as shown in §8.5.8), rather than simply
using cin. Also give the Token_stream constructor (§7.8.2) an istream&
parameter so that when we figure out how to make our own istreams
(e.g., attached to files), we can use the calculator for those. Hint:
Don’t try to copy an istream.
I'd say this is the code that might relate to what I have to do here?
class Token_stream {
public:
Token get(); // get a Token
void putback(Token t); // put a token back
void ignore(char c); // discard characters up to and including a c
private:
bool full{ false }; // is there a Token in the buffer?
Token buffer{ '0' };
// here is where putback() stores a Token
};
Token Token_stream::get()
{
if (full) { // do we already have a Token?
full = false; // remove Token from buffer
return buffer;
}
char ch;
cin.get(ch); // look for any char including whitespace
while (isspace(ch) && ch != '\n') cin.get(ch);
switch (ch) {
case '\n':
return Token{ print };
case print:
case quit:
case help:
case '(':
case ')':
case '{':
case '}':
case '!':
case '+':
case '-':
case '*':
case '/':
case '%':
case '=':
case ',':
return Token{ ch }; // let each character represent itself
case '.':
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9':
{
cin.putback(ch); // put digit back into input stream
double val;
cin >> val; // read floating-point number
return Token{ number, val };
}
default:
if (isalpha(ch)) {
string s;
s += ch;
while (cin.get(ch) &&
((isalpha(ch) || isdigit(ch) || ch == '_')))
s += ch;
cin.putback(ch);
if (s == declkey) return Token{ let }; // declaration keyword
else if (s == sqrtkey) return Token{ square_root };
else if (s == expkey) return Token{ exponent };
else if (s == sinkey) return Token{ c_sin };
else if (s == coskey) return Token{ c_cos };
else if (s == quitkey) return Token{ quit };
else if (s == helpkey) return Token{ help };
else return Token{ name, s };
}
error("Bad token");
}
};
But I'm still not really sure what I have to do here? Like I just don't understand what I need to do and how.. Can anybody maybe help me out? I did some research on what istream is but I still am not really sure what to do here..

Answer 1

He means to give token to function as an argument rather than using a std::cin statement inside a function.
void Token_stream::putback(Token t) //like this one
{
if (full) error("putback() into full buffer");
buffer = t;
full = true;
}
// ignore Tokens up to specific kind
void Token_stream::ignore(char c) // and this one
{
// first look in buffer:
if (full && c==buffer.kind) {
full = false;
return;
}
full = false;
char ch = 0;
while (cin>>ch)
if (ch==c) return;
}