变量、类型与类型安全

#include <iostream>
#include <cstdint>   // Fixed-width integer types
#include <limits>    // std::numeric_limits

int main() {
    // Boolean
    bool is_valid = true;           // true or false, typically 1 byte

    // Character types
    char letter = 'A';             // At least 8 bits, holds ASCII
    char8_t  u8ch  = u8'A';       // C++20: exactly 8 bits, UTF-8
    char16_t u16ch = u'\u00E9';   // At least 16 bits, UTF-16
    char32_t u32ch = U'\U0001F600'; // At least 32 bits, UTF-32

    // Integer types (sizes are minimums, typical x86-64 shown)
    short       s = 32000;         // At least 16 bits (16)
    int         i = 2'000'000;     // At least 16 bits (32) — digit separator C++14
    long        l = 100'000L;      // At least 32 bits (64 on Linux, 32 on Windows)
    long long  ll = 9'000'000'000'000'000'000LL;  // At least 64 bits

    // Unsigned variants
    unsigned int ui = 42u;         // Cannot be negative

    // Floating-point types
    float       f = 3.14f;         // ~7 decimal digits precision
    double      d = 3.141592653589793; // ~15 decimal digits precision
    long double ld = 3.14159265358979323846L;

    // Fixed-width types from <cstdint> — use these when size matters
    std::int32_t  exact = 42;      // Exactly 32 bits, guaranteed
    std::uint64_t big   = 18'446'744'073'709'551'615ULL; // Exactly 64 bits
    std::int_fast32_t fast = 42;   // At least 32 bits, fastest available
    std::size_t index = 0;         // Unsigned, suitable for sizes and indices

    std::cout << "int: " << sizeof(int) << " bytes, max = "
              << std::numeric_limits<int>::max() << '\n';
}

#include <iostream>
#include <string>

int main() {
    // Direct initialization
    int a(42);              // OK but can be confused with function declaration

    // Copy initialization
    int b = 42;             // OK, familiar from C

    // Uniform (brace) initialization — preferred in modern C++
    int c{42};              // Direct-list-initialization
    int d = {42};           // Copy-list-initialization

    // Brace initialization PREVENTS narrowing conversions
    // double to int loses precision — the compiler will ERROR:
    // int narrow{3.14};    // ERROR: narrowing conversion
    int ok = 3.14;          // Compiles! Silently truncates to 3 (dangerous)

    // Value initialization (zero for built-in types)
    int zero{};             // Guaranteed to be 0
    double dzero{};         // Guaranteed to be 0.0
    bool bfalse{};          // Guaranteed to be false

    // auto deduction
    auto x = 42;            // int
    auto y = 3.14;          // double
    auto z = 'A';           // char
    auto s = std::string{"hello"};  // std::string (not const char*!)

    // decltype — queries the type of an expression
    decltype(x) another = 100;  // same type as x → int
    decltype(auto) ref = (x);   // int& because (x) is an lvalue expression

    std::cout << "zero-initialized int: " << zero << '\n';
    std::cout << "auto-deduced string: " << s << '\n';
}