Data Validation in C++: Libraries and Methods (libvalidate, nlohmann/json, Boost)

Data Validation in C++: Overview of Libraries and Practical Approaches

Data validation is one of the key aspects of developing reliable software in C++. Correct input data checking helps prevent runtime errors, vulnerabilities, and undefined behavior. Unlike dynamically typed languages, C++ requires explicit control over types and formats from the developer. In this article, we will look at both manual methods and specialized libraries that simplify validation.

1. Manual Validation: Basic Techniques

The simplest way is to check data using conditional statements. This approach is good for small projects but quickly becomes cumbersome.

#include <iostream>#include <string>#include <cctype>

bool validateEmail(const std::string& email) {    if (email.empty()) return false;    size_t atPos = email.find('@');    if (atPos == std::string::npos || atPos == 0) return false;    size_t dotPos = email.rfind('.');    if (dotPos == std::string::npos || dotPos <= atPos + 1) return false;    if (dotPos == email.length() - 1) return false;    return true;}

int main() {    std::string testEmail = "user@example.com";    std::cout << (validateEmail(testEmail) ? "Valid" : "Invalid") << std::endl;    return 0;}

Disadvantages: maintenance complexity, code duplication, lack of a unified interface.

2. The libvalidate Library

libvalidate is a lightweight library for declarative data validation in C++17 and later. It allows you to describe validation rules in a chained style reminiscent of validators in PHP or Python.

Installation

The library is distributed as header-only. Simply copy validate.hpp into your project or include it via CMake:

# Example FetchContentinclude(FetchContent)FetchContent_Declare(libvalidate    GIT_REPOSITORY https://github.com/username/libvalidate.git    GIT_TAG v1.0.0)FetchContent_MakeAvailable(libvalidate)

Usage Example

#include "validate.hpp"#include <iostream>#include <string>

int main() {    using namespace validate;

    // Number validation (age)    int age = 25;    auto ageResult = check(age)        .min(18, "Age must be at least 18")        .max(120, "Age must be no more than 120");

    if (!ageResult.valid()) {        std::cerr << ageResult.error() << std::endl;    }

    // String validation (password)    std::string password = "P@ssw0rd";    auto passResult = check(password)        .minLength(8, "Password must contain at least 8 characters")        .matches(R"((?=.*[A-Z])(?=.*[a-z])(?=.*\\d).*)", "Password must contain uppercase, lowercase letters and digits");

    if (!passResult.valid()) {        std::cerr << passResult.error() << std::endl;    }

    // Combined object validation    struct User {        std::string name;        int age;        std::string email;    };

    User user{"Alice", 30, "alice@example.com"};    auto userResult = check(user)        .field(&User::name, "name")            .minLength(2)            .maxLength(50)        .field(&User::age, "age")            .range(18, 100)        .field(&User::email, "email")            .email(); // built-in email validation

    if (!userResult.valid()) {        for (const auto& err : userResult.errors()) {            std::cerr << err.field << ": " << err.message << std::endl;        }    }

    return 0;}

Features:

• Support for built-in rules: min, max, range, email, regex, notEmpty.
• Ability to create