What Is the Unsigned Length and Subscript Problem?

The C++ standard library uses unsigned types for container lengths and indices, which creates friction with the general preference for signed integers. This lesson explores the practical implications and how to work with them safely.

Working with std::vector involves three primary operations: getting the length, accessing elements by index, and traversing the container. Each operation has type considerations you need to understand.

The historical unsigned decision

When the C++ standard library was designed, array lengths and indices were made unsigned. The reasoning included:

  • Negative indices don't make logical sense for standard arrays
  • Unsigned types provide one extra bit of range (important when 16-bit systems were common)
  • Validation could skip checking for negative values

Modern perspective recognizes issues with this choice. Unsigned types don't prevent negative values—implicit conversion simply wraps negative signed integers to large positive unsigned values. Modern 32/64-bit systems rarely need that extra bit, and operator[] doesn't bounds-check anyway.

We generally prefer signed integers for quantities, but standard library containers use unsigned types, creating friction.

Understanding size_type in container classes

Standard library containers define a nested type alias size_type for lengths and indexing:

#include <iostream>
#include <vector>

int main()
{
    std::vector<double> prices{ 19.99, 24.50, 15.75 };

    std::vector<double>::size_type count{ prices.size() };
    std::cout << "We have " << count << " items\n";

    return 0;
}

For std::vector (and nearly all standard containers), size_type aliases std::size_t, a large unsigned integral type. While technically customizable for specific allocators, you can safely assume it's std::size_t.

Getting the length of a vector

Three approaches:

Option 1: The size() member function

#include <iostream>
#include <vector>

int main()
{
    std::vector scores{ 85, 92, 78, 90, 88 };
    std::cout << "Recorded " << scores.size() << " scores\n";

    return 0;
}

Returns unsigned size_type.

Option 2: The std::size() non-member function (C++17)

#include <iostream>
#include <vector>

int main()
{
    std::vector scores{ 85, 92, 78, 90, 88 };
    std::cout << "Recorded " << std::size(scores) << " scores\n";

    return 0;
}

Calls the size() member function, returning unsigned size_type.

Option 3: The std::ssize() function (C++20)

#include <iostream>
#include <vector>

int main()
{
    std::vector scores{ 85, 92, 78, 90, 88 };
    std::cout << "Recorded " << std::ssize(scores) << " scores\n";

    return 0;
}

Returns a large signed integral type (typically std::ptrdiff_t)—the only standard option giving a signed length.

To store the length in a signed variable with the first two options, use static_cast:

#include <vector>

int main()
{
    std::vector items{ 10, 20, 30, 40, 50 };
    int count{ static_cast<int>(items.size()) };

    return 0;
}

Accessing elements: operator[] without bounds checking

The subscript operator provides fast, unchecked access:

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> levels{ 5, 12, 8, 15, 20 };

    std::cout << levels[2] << '\n';  // Prints 8
    std::cout << levels[10] << '\n'; // Undefined behavior!

    return 0;
}

No validation occurs. Invalid indices trigger undefined behavior—crashes, garbage output, or apparently correct behavior (most dangerous).

Accessing elements: at() with runtime bounds checking

For validated access, use at():

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> levels{ 5, 12, 8, 15, 20 };

    std::cout << levels.at(2) << '\n';  // Prints 8
    std::cout << levels.at(10) << '\n'; // Throws std::out_of_range

    return 0;
}

When at() encounters invalid indices, it throws std::out_of_range. Unhandled, this terminates cleanly rather than causing undefined behavior.

While safer, at() has performance overhead from bounds checking on every call. Most code uses operator[] with prior validation.

Constexpr indices avoid narrowing conversion issues

When using constexpr signed values as indices, the compiler verifies safe conversion:

#include <iostream>
#include <vector>

int main()
{
    std::vector ranks{ "Bronze", "Silver", "Gold", "Platinum", "Diamond" };

    std::cout << ranks[2] << '\n';          // OK: literal 2 converts safely

    constexpr int tier{ 3 };
    std::cout << ranks[tier] << '\n';       // OK: constexpr value converts safely

    return 0;
}

Since the compiler verifies at compile-time that values are non-negative and representable, these conversions aren't narrowing.

Non-constexpr signed indices create warnings

Problems arise with non-constexpr signed indices:

#include <iostream>
#include <vector>

int main()
{
    std::vector slots{ "Slot A", "Slot B", "Slot C", "Slot D", "Slot E" };

    int position{ 3 };
    std::cout << slots[position] << '\n'; // Possible warning: narrowing conversion

    return 0;
}

Even though position is positive and valid, runtime conversion from int to std::size_t is narrowing, potentially triggering warnings.

Resolution: use std::size_t for index variables:

#include <iostream>
#include <vector>

int main()
{
    std::vector slots{ "Slot A", "Slot B", "Slot C", "Slot D", "Slot E" };

    std::size_t position{ 3 };
    std::cout << slots[position] << '\n'; // No conversion needed

    return 0;
}

Alternative: index the underlying array via data():

#include <iostream>
#include <vector>

int main()
{
    std::vector slots{ "Slot A", "Slot B", "Slot C", "Slot D", "Slot E" };

    int position{ 3 };
    std::cout << slots.data()[position] << '\n'; // C-style arrays accept signed indices

    return 0;
}

Since C-style arrays support both signed and unsigned indexing, this avoids conversion warnings.

Summary

Historical context: Standard library designers chose unsigned types for array lengths and indices. Modern understanding considers this problematic since implicit conversions don't actually prevent negative values from being used.

size_type typedef: Containers define size_type for lengths and indices, which is std::size_t for std::vector—a large unsigned integral type.

Three ways to get length: size() member function, std::size() (C++17), and std::ssize() (C++20). Only std::ssize() returns a signed type.

operator[] vs at(): operator[] is fast but performs no bounds checking—invalid indices cause undefined behavior. at() performs runtime bounds checking and throws std::out_of_range for invalid indices.

Constexpr indices: When indices are constexpr, the compiler verifies conversions are safe, avoiding narrowing warnings. Non-constexpr signed indices may trigger warnings.

Workarounds: Use std::size_t for index variables, or use data() to access the underlying C-style array which accepts signed indices.

The unsigned length and subscript issue is an unavoidable friction point when working with standard library containers. Understanding the trade-offs helps you write cleaner code and avoid common pitfalls.