Reading and Validating Input in C and C++

What is Reading and Validating Input?

Input functions can fail, so production code must test their result instead of assuming valid data.

Important points

Compile with warnings enabled and fix every warning.
Know the lifetime and type of every value.
Validate input and keep array bounds explicit.

C and C++ code examples

C

main.c

#include <stdio.h>

int main(void) {
    int age;
    printf("Age: ");
    if (scanf("%d", &age) != 1 || age < 0) {
        puts("Invalid age");
        return 1;
    }
    printf("Next year: %d\n", age + 1);
    return 0;
}

Result of running the code

Age: 20
Next year: 21

C++

Run code →

main.cpp

#include <iostream>

int main() {
    int age;
    std::cout << "Age: ";
    if (!(std::cin >> age) || age < 0) {
        std::cerr << "Invalid age\n";
        return 1;
    }
    std::cout << "Next year: " << age + 1 << '\n';
}

Result of running the code

Age: 20
Next year: 21

C and C++ comparison

scanf returns the number of successfully converted fields. std::cin enters a failed state when extraction cannot satisfy the requested type.

C

C exposes small procedural APIs and makes representation details explicit.

C++

C++ retains the low-level model while adding safer library types and abstractions.

Practice exercises

Run both versions, then modify them to observe the different language guarantees.

Add invalid and boundary-value inputs.
Compile with -Wall -Wextra -Wpedantic.
Move reusable declarations into a header and implementation file.

Practice with a challenge

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