How to use Parametter Template

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SPyofgame's blog

How to use Parametter Template

By SPyofgame, history, 7 years ago, In English

I dont know how to use Template for multiple input. Can you help me ?

Fast Input Code

template <typename T>
bool read_int(T &x) {
    bool start = false , neg = false;
    T ret = 0;
    for (char r; r = getchar(); ) { 
        if (r == '-') neg = true;
        else if ((r < '0' || r > '9'))
            if (start) return false; else continue;
        else 
            ret += r - '0';

        if (start) ret *=10;
        start = true;
    }   
    return x = (neg ? -ret : ret), true;
}
template <typename T, typename... Tnext> 
bool read_int(T &x, Tnext... next)
{
    read_int(next...);
}

Sorry for my bad English. Correct me if I am wrong.

Thanks for reading <3

#template, #cpp

SPyofgame
7 years ago
13

Comments (12)

Show archived | Write comment?

GLAYS

7 years ago, hide # |

+10

For versions >= C++14

template <typename T>
bool read_int1(T &x) {
    bool start = false , neg = false;
    T ret = 0;
    for (char r; r = getchar(); ) { 
        if (r == '-') neg = true;
        else if ((r < '0' || r > '9'))
            if (start) return false; else continue;
        else 
            ret += r - '0';

        if (start) ret *=10;
        start = true;
    }   
    return x = (neg ? -ret : ret), true;
}

template <typename... T>
void read_int(T& ... x){
	((read_int1(x)), ...);
}

Or like this

template <typename T>
bool read_int(T &x) {
    bool start = false , neg = false;
    T ret = 0;
    for (char r; r = getchar(); ) { 
        if (r == '-') neg = true;
        else if ((r < '0' || r > '9'))
            if (start) return false; else continue;
        else 
            ret += r - '0';

        if (start) ret *=10;
        start = true;
    }   
    return x = (neg ? -ret : ret), true;
}

template <typename T, typename ... U>
void read_int(T& t, U& ... u){
	read_int(t);
	read_int(u ...);
}

→ Reply

SPyofgame

7 years ago, hide # ^ |

Int the first template:

My test code

#include <iostream>

using namespace std;

template <typename T>
bool read_int1(T &x) {
    bool start = false , neg = false;
    T ret = 0;
    for (char r; r = getchar(); ) { 
        if (r == '-') neg = true;
        else if ((r < '0' || r > '9'))
            if (start) return false; else continue;
        else 
            ret += r - '0';

        if (start) ret *=10;
        start = true;
    }   
    return x = (neg ? -ret : ret), true;
}

template <typename... T>
void read_int(T& ... x){
	((read_int1(x)), ...);
}

int main()
{
    int a, b, c;
    read_int(a, b, c);
    cout << a << ' ' << b << ' ' << c;
    return 0;
}

It shows in the terminal

/home/spyofgame/Desktop/learning/cpp/src/main.cpp: In function ‘void read_int(T& ...)’:
/home/spyofgame/Desktop/learning/cpp/src/main.cpp:24:19: warning: fold-expressions only available with -std=c++1z or -std=gnu++1z
  ((read_int1(x)), ...);
                   ^~~

Input

1 2 3

Output

22035 728251024 32765

→ Reply

SPyofgame

7 years ago, hide # ^ |

Int the second template:

My test code

#include <iostream>

using namespace std;

template <typename T>
bool read_int(T &x) {
    bool start = false , neg = false;
    T ret = 0;
    for (char r; r = getchar(); ) { 
        if (r == '-') neg = true;
        else if ((r < '0' || r > '9'))
            if (start) return false; else continue;
        else 
            ret += r - '0';

        if (start) ret *=10;
        start = true;
    }   
    return x = (neg ? -ret : ret), true;
}

template <typename T, typename ... U>
void read_int(T& t, U& ... u){
	read_int(t);
	read_int(u ...);
}

int main()
{
    int a, b, c;
    read_int(a, b, c);
    cout << a << ' ' << b << ' ' << c;
    return 0;
}

It shows in the terminal

None

Input

1 2 3

Output

21858 -181756944 32765

→ Reply

SPyofgame

7 years ago, hide # ^ |

What happened ;-; ? I use vscode

→ Reply

SPyofgame

7 years ago, hide # ^ |

Run code in CUSTOM TEST doesnt work too

→ Reply

GLAYS

7 years ago, hide # |

The bug is in your original read_int

Both of these work fine.

First template

template <typename... T>
void read_int(T& ... x){
	((cin>>x), ...);
}

Second template

template <typename T>
bool read_int(T &x) {
    cin>>x;
    return true;
}

template <typename T, typename ... U>
void read_int(T& t, U& ... u){
	read_int(t);
	read_int(u ...);
}

→ Reply

SPyofgame

7 years ago, hide # ^ |

Yes they are <3 Thanks

→ Reply

SPyofgame

7 years ago, hide # ^ |

But how can I detect if there is no valid input ?

→ Reply

GLAYS

7 years ago, hide # ^ |

← Rev. 2 →

You're welcome!

I don't know how to manipulate that function, I believe cin/cout with sync is enough.

If you meant how to read until the input ends, it's done like this:

string s;
while(cin >> s){
    ...
}

→ Reply

SPyofgame

7 years ago, hide # ^ |

← Rev. 2 →

Yes I know about it, but I mean to the template of custom cin >> operator

→ Reply

SPyofgame

7 years ago, hide # ^ |

well... actually I have just meet a problem where cin_with_sync is not enough :) And this is the first problem I need to use FastInput, I was surprised

→ Reply

SPyofgame

7 years ago, hide # ^ |

How can I check if there is no more arguments in parametter ?

→ Reply

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