Day 7 — Type Aliases, enum class, and Nested Templates

1. Problem It Solves

Type aliases shorten meaningful type names, enum class creates scoped enumerators, and nested templates describe containers of containers without lexical ambiguity. It makes an important constraint visible instead of leaving readers to guess. This lesson keeps only the C++11 core that fits one focused day.

2. Prerequisites

The ideas from Day 6, plus basic variables, functions, and output already introduced.

3. Core Idea

Mental model: Type aliases shorten meaningful type names, enum class creates scoped enumerators, and nested templates describe containers of containers without lexical ambiguity. Identify the relevant value or state, who owns it, and whether the rule acts during compilation or execution.

4. Minimal Syntax

using Scores = std::vector<int>; enum class State { ready, busy };

5. How It Works

The example creates a tiny fixed state with no keyboard input.
C++11 or the standard-library contract applies today's rule.
The program prints the important result so it can be checked against the source.

6. Common Mistakes

An enum class value does not implicitly convert to int; forcing old enum assumptions causes a compile error rather than a silent conversion.

7. When to Use It

Use it when a long type needs a domain name or enumerators must not leak into the surrounding scope.
Avoid it when it hides ownership, lifetime, type, ordering, or cost.

8. Simple Example

The program names a score vector, uses a scoped state, and stores a tiny two-dimensional grid as a nested std::vector. The .cpp keeps the data fixed and avoids unrelated abstraction.

Complete sample code

Source file

cpp11/07_type_aliases_enum_class_nested_templates/main.cpp

C++

#include <iostream>
#include <vector>

using Scores = std::vector<int>;

enum class State { ready, busy };

int main() {
    Scores scores{7, 8, 9};
    std::vector<std::vector<int> > grid{{1, 2}, {3, 4}};
    State state = State::ready;

    std::cout << "score=" << scores[1] << '\n';
    std::cout << "cell=" << grid[1][0] << '\n';
    std::cout << "ready=" << (state == State::ready) << '\n';
}

9. Key Takeaways

The feature is part of the C++11 scope used in this course.
Understand its lifetime, ownership, type, and ordering consequences.
Compile with warnings and prefer the smallest form that makes the rule obvious.

10. Self-Check Questions

Easy — How do using Scores = std::vector<int> and enum class State { ready, busy } clarify type declarations? Why write State::ready?
Medium — Read the small example described above. What value or state should it print, and which rule produces that result?
Hard — Find and explain the subtle bug in this situation: An enum class value does not implicitly convert to int; forcing old enum assumptions causes a compile error rather than a silent conversion. What is the smallest C++11-safe correction?

Practice this lesson's cards

1. Problem It Solves

#include <iostream> #include <vector> using Scores = std::vector<int>; enum class State { ready, busy }; int main() { Scores scores{7, 8, 9}; std::vector<std::vector<int> > grid{{1, 2}, {3, 4}}; State state = State::ready; std::cout << "score=" << scores[1] << '\n'; std::cout << "cell=" << grid[1][0] << '\n'; std::cout << "ready=" << (state == State::ready) << '\n'; }

10. Self-Check Questions

Easy — How do using Scores = std::vector<int> and enum class State { ready, busy } clarify type declarations? Why write State::ready?
Medium — Read the small example described above. What value or state should it print, and which rule produces that result?
Hard — Find and explain the subtle bug in this situation: An enum class value does not implicitly convert to int; forcing old enum assumptions causes a compile error rather than a silent conversion. What is the smallest C++11-safe correction?