Member Avatar for Jon182

Hey guys, I was just wondering if there is a way in C++ to stop a program from crashing if the user enters a char instead of an int by mistake?

Thanks in advance.

  • 3 Contributors
  • 7 Replies
  • 462 Views
  • 6 Hours Discussion Span
  • Latest Post Latest Post by Jon182

Dani AI

Generated

A short, practical summary and a couple of safe patterns to avoid the “char entered where an int was expected” problem.

As pointed out, formatted extraction (the >> operator) puts the stream into a failed state when it cannot parse the expected numeric text; the bad characters remain in the buffer and future extractions keep failing. That’s why a simple menu loop can appear to “hang.” See the standard behavior of formatted extraction for details. (en.cppreference.com)

A modern, robust approach is: read a whole line, validate/parse it, then only use the value if parsing succeeded. For performance and no-throw parsing use C++17’s std::from_chars (locale-independent, non-throwing). Example:

#include <charconv>
#include <string>

bool parse_int(const std::string &s, int &out) {
    auto res = std::from_chars(s.data(), s.data() + s.size(), out);
    return res.ec == std::errc() && res.ptr == s.data() + s.size();
}

Note: std::from_chars does not skip leading whitespace, so trim or use getline first. See the std::from_chars docs. (en.cppreference.com)

If you prefer simplicity over zero-allocation parsing, std::stoi is convenient but throws exceptions on invalid input or out-of-range values, so wrap it in try/catch and check the returned position to detect trailing junk:

size_t pos;
int v = std::stoi(line, &pos, 10);
if (pos != line.size()) /* trailing non-digit chars */;

std::stoi throws std::invalid_argument or std::out_of_range on failure. (my.eng.utah.edu)

If you’re still using MSVC6 (Visual Studio 6.0 from 1998, as mentioned), many C++11/17 facilities aren’t available; either stick to careful manual validation on input or upgrade to a modern compiler to use the safer APIs. (news.microsoft.com)

Final note: prefer “read-then-validate-then-convert” over blind formatted extraction. It avoids silent failures, makes error messages specific, and keeps loops from reprocessing the same bad input.

Recommended Answers

All 7 Replies

Member Avatar for Drowzee

The foolproof way is to take a string input (chars) and then parse the input character by character to determine if the input is valid through use of the functions in the <ctype> library.

int isdigit(char c);
int isalpha(char c);

Those return 1 if true, and 0 if not.

Member Avatar for Jon182

Thank you for the quick reply.

Member Avatar for Drowzee

Thank you for the quick reply.

No problem... You'll still have to to a conversion between the character digits and integers. Just subtract '0' (the character zero) from the character digit, and it'll give the correct integer result.

Edit:
Or, if you're dealing with double-digit(or more) numbers, you might want to first check for the valid input then use the

int atoi(const char*s);

from <stdlib> to get your character string converted to the correct integer.

Member Avatar for Narue

>stop a program from crashing if the user enters a char instead of an int by mistake?
The program shouldn't crash simply by reading the wrong type. Now, when you actually try to use the value you'll have issues if the variable wasn't initialized to something predictable. Post your code, because this problem usually must be solved on a case by case basis. However, there are two options for avoiding it:

1) Loop on failure:

int val;

while ( !( cin>> val ) ) {
  cerr<<"Invalid input, try again: ";
  cin.clear();
  cin.ignore ( numeric_limits<streamsize>::max(), '\n' );
}

2) Only use string input (there are many variations):

string line;
int val;

getline ( cin, line );
val = static_cast<int> ( strtol ( line.c_str(), 0, 0 ) );
Member Avatar for Jon182

Thanks for the replys I don't actually have any code at the moment but I was working on a program a few months ago (i'm only a beginner) I was using MSVC6 to create the program which had a menu so that the user could enter a number for what they wanted to do. When testing the program I hit a character by mistake and the program just seemed to go into an endless loop.

I am trying to improve my C++ so I will set myself a task soon so I can try out your ideas.

Thanks again.

Member Avatar for Narue

>I hit a character by mistake and the program just seemed to go into an endless loop.
Then you want the first option. Because cin expects a valid number, and what you typed was not a valid number, it was never extracted from the stream and cin keeps failing on it. So you need to detect that cin failed, then clear any stream errors, and finally remove the faulty input so you can try again.

>int atoi(const char*s);
Don't ever use this function unless you're sure the string will be converted without fail. atoi has undefined behavior when it can't convert the string.

Member Avatar for Jon182

Thank you very much for your advice.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.