Disabling Implicit Casting in c++

Question

Is it possible to disable implicit casting in C/C++.

Suppose I want to write a validity function that makes me enter only integers in range [1,10]

I have written:

#include <iostream>


using namespace std;

int main( )
{
    int var=0;
    cout << "Enter a number (Integer between 1 to 10) : ";

    while( (!(cin >> var )) || (var > 10 ) || (var < 1) )
    {
        cout << "U nuts .. It can only be [1,10]\n";
        cin.clear();
        cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
        cout << "Enter a number (Integer between 1 to 10) : ";
        }

    cout << "\nYou entered : " << var;

   return 0;
}

But if the user enters 9.5 it accepts it by converting the float 9.5 as 9 and storing it in var. I want any float entry to be treated as invalid entry. How do I achieve this most compactly.

I do not want to do something of this sort:

#include <iostream>
#include <cmath>


using namespace std;

int main( )
{
    float var=0;
    cout << "Enter a number (Integer between 1 to 10) : ";

    while( (!(cin >> var )) ||(var < 1)|| (var > 10 ) || !(ceilf(var) == var) )
    {
        cout << "U nuts .. It can only be [1,10]\n";
        cin.clear();
        cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
        cout << "Enter a number (Integer between 1 to 10) : ";
        }

    cout << "\nYou entered : " << var;

   return 0;
}

This serves my purpose . But what I want to know, that is there any way where a conversion from float to int - I can suppress or it can show it as false input.

Similar to the way cin >> var where var type is int - if we enter char it returns false condition. Can we achieve the same for float entry ?

Thanks

`while( (!(cin >> var )` what do you think this is doing? checking if var is 0 or not or if cin is null or not?. this will always evaluate to false. — Koushik Shetty, Apr 29 '13 at 11:00
@Koushik - if u enter 'char' then 'cin >> var' returns false as var is of type - int. — Gaurav K, Apr 29 '13 at 11:02
Gaurav See [this](http://stackoverflow.com/questions/7903083/how-do-i-prevent-an-implicit-cast-double-int), it talks about preventing `double` to `int` implicit conversion — Suvarna Pattayil, Apr 29 '13 at 11:03
For checking floating point number entries.. this may get handy.. http://cboard.cprogramming.com/c-programming/71064-checking-floating-point-number.html — Hiren Pandya, Apr 29 '13 at 11:05
cin >> var' returns false? how is that? `>>` is overloaded for istream and it return `istream&`. if you enter char its converted to int(implicitly and stored in var). — Koushik Shetty, Apr 29 '13 at 11:05
@Koushik dude .. check this out .. u got confusion with I/O Stream Class ... http://www.parashift.com/c++-faq-lite/istream-and-ignore.html — Gaurav K, Apr 29 '13 at 11:07
@Koushik He means if you enter the string "char", not a `char` value. Then yes, the fail bit will be set and the stream, when converted to `bool`, will be `false`. — Joseph Mansfield, Apr 29 '13 at 11:11
@sftrabbit, He is confused as to how the implicit conversion of `istream&` to `boolean` takes place. In his estimation, the ASCII value of `char` will be stored — Suvarna Pattayil, Apr 29 '13 at 11:16
@sftrabbit if you enter any character - suppose I enter `'j'` or `'hello'` - then for expression `'cin >> var'` fail bit will be set for returned `istream&` - and it evalutes to `false` as `var` type is `int` — Gaurav K, Apr 29 '13 at 11:16
@Koushik Check [this](http://stackoverflow.com/questions/6791520/if-cin-x-as-a-condition) too — Suvarna Pattayil, Apr 29 '13 at 11:19
@SuvP is there a conversion operator (operator bool()?)defined for istream? — Koushik Shetty, Apr 29 '13 at 11:21
@SuvP the conversion is not implicit but through conversion operator. and yes i was really confused about accepting char. i thought since char is integral type it will be promoted to int and stored. but wow learnt. Gaurav thanks and sorry. — Koushik Shetty, Apr 29 '13 at 11:27
@Koushik in C++11 yes there is an `operator bool()`, in C++98 there is an `operator void*` that is compared to `null` in a boolean expression. — zakinster, Apr 29 '13 at 11:29
@zakinster how is the call resolved to void * coversion operator in pre c++11 era? — Koushik Shetty, Apr 29 '13 at 11:31
@Koushik Because it's the only implicit conversion that would make the object type valid in this context. See this : http://stackoverflow.com/questions/6277678/stream-output-and-implicit-void-cast-operator-function-invocation — zakinster, Apr 29 '13 at 11:34
@Koushik. Yeps. I was searchin for a nice link . But I guess its all cleared now :) — Suvarna Pattayil, Apr 29 '13 at 11:39
@SuvP i never realised istream having conversion operator thats why the whole mess. better late than never. thanks a lot:-) . — Koushik Shetty, Apr 29 '13 at 11:43
Note that there is no such thing as "implicit casting". A **cast** is something you write in your code to tell the compiler to do a **conversion**. An **implicit conversion** is a conversion that the compiler does without a cast; an **explicit conversion** is one that the compiler does because of a cast. — Pete Becker, Apr 29 '13 at 12:22

Steve Jessop · Accepted Answer · 2013-04-29T12:59:37.923

6

But if the user enters 9.5 it accepts it by converting the float 9.5 as 9 and storing it in var.

No it doesn't. If the the user enters 9.5, then cin >> var stops reading when it hits the . (and leaves it on the stream). There's no float-to-int conversion because you haven't read a float, just an int.

The fix is to read the rest of the input (after cin >> var), and make sure there's nothing bad left over after the end of the int.

edited Apr 29 '13 at 12:59

answered Apr 29 '13 at 11:40

Steve Jessop

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What is the best way to check the residual characters in cin stream ? Is there any function available to check residual? Also `cin` always confuses me - suppose it is taking an string entry.. when we hit `enter` does it remain in the stream or extracted and saved in string var `str` for expressions such as `cin >> str` – Gaurav K Apr 29 '13 at 11:52
@GauravK could try [cin.ignore](http://en.cppreference.com/w/cpp/io/basic_istream/ignore) after you read to var. – Koushik Shetty Apr 29 '13 at 11:58
added to previous comment : What I mean to ask - `string str; cin >> str` now I enter `hello` followed by hitting `enter` - does this `enter` remain in the stream or it is also saved to `str` along with `hello` – Gaurav K Apr 29 '13 at 11:59
@Koushik ignore will discard residuals , I wana check if any residual is there or not ? as explained in the answer - when we enter `9.5` , `.5` will remain in the cin stream - is there any method to check residual in istream class object? – Gaurav K Apr 29 '13 at 12:02
@GauravK: the best way is `string str; readline(cin, str);`. In the case where the user enters `9` that will read an empty string, whereas `cin >> str` would read the next line. – Steve Jessop Apr 29 '13 at 12:57
@SteveJessop can you please give an example ? – Gaurav K Apr 29 '13 at 13:04
@Gaurav `9.5` would be left in the buffer. – Suvarna Pattayil Apr 29 '13 at 13:55

score 1 · Answer 2 · answered Apr 29 '13 at 11:04

1

If you want to validate all of the input, you will have to get the whole line first.

Try:

string line;
getline(cin, line); // gets whole line
char *endptr;
long int var = strtol(line.c_str(), &endptr, 10); // converts string to int

// now check that endptr points to end of the string
if (endptr<line.c_str()+line.length()) {
  // extra characters found after the integer
}

answered Apr 29 '13 at 11:04

ronalchn

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This depends on the desired behaviour, and I am not in a position to determine that. The OP will be most qualified in determining the tweaks necessary, and it is not difficult to achieve this, so I will not add this behaviour to the answer. – ronalchn Apr 29 '13 at 11:18

Disabling Implicit Casting in c++

2 Answers2