函数与参数传递

#include <iostream>
#include <string>
#include <vector>

// 1. Pass by VALUE — the function gets a copy
//    Use for: small types (int, double, char), when you need a local copy
void square(int x) {           // x is a copy of the argument
    x = x * x;                 // Modifying x does NOT affect the caller
    std::cout << "Inside: " << x << '\n';
}

// 2. Pass by REFERENCE — the function works on the original
//    Use for: when you need to modify the caller's variable
void double_it(int& x) {       // x IS the caller's variable
    x *= 2;                    // This DOES modify the caller's variable
}

// 3. Pass by CONST REFERENCE — read-only access to the original, no copy
//    Use for: large objects you only need to read
void print_all(const std::vector<std::string>& names) {
    for (const auto& name : names) {
        std::cout << name << '\n';
    }
    // names.push_back("Eve");  // ERROR: names is const
}

// 4. Pass by POINTER — explicit nullable indirection
//    Use for: optional parameters (can be nullptr), C API interop
void maybe_increment(int* ptr) {
    if (ptr != nullptr) {       // Must check for null!
        ++(*ptr);
    }
}

int main() {
    int val = 5;

    square(val);
    std::cout << "After square: " << val << '\n';      // Still 5 (copied)

    double_it(val);
    std::cout << "After double_it: " << val << '\n';   // Now 10 (modified)

    std::vector<std::string> people = {"Alice", "Bob", "Charlie"};
    print_all(people);         // No copy of the vector — efficient

    maybe_increment(&val);     // Pass address explicitly
    std::cout << "After maybe_increment: " << val << '\n';  // 11

    maybe_increment(nullptr);  // Safe: the function checks for null
}

#include <iostream>
#include <string>
#include <vector>

// Function overloading — same name, different parameter types/counts
void log(int value) {
    std::cout << "[INT] " << value << '\n';
}

void log(double value) {
    std::cout << "[DOUBLE] " << value << '\n';
}

void log(const std::string& value) {
    std::cout << "[STRING] " << value << '\n';
}

// Default arguments — provide fallback values
void greet(const std::string& name, const std::string& greeting = "Hello") {
    std::cout << greeting << ", " << name << "!\n";
}

// [[nodiscard]] (C++17) — warn if return value is ignored
[[nodiscard]] int compute(int a, int b) {
    return a * b + a;
}

// [[nodiscard]] with reason (C++20)
[[nodiscard("Ignoring error codes can hide failures")]]
int open_file(const std::string& path) {
    // Returns 0 on success, negative on error
    return 0;  // Simplified
}

// inline — suggests the compiler inline this function (also allows
// the function to be defined in a header without ODR violations)
inline int fast_max(int a, int b) {
    return (a > b) ? a : b;
}

int main() {
    log(42);               // Calls log(int)
    log(3.14);             // Calls log(double)
    log(std::string{"hi"});// Calls log(const string&)

    greet("Alice");                // Uses default: "Hello, Alice!"
    greet("Bob", "Good morning"); // Uses provided: "Good morning, Bob!"

    // int result = compute(3, 4);  // OK
    // compute(3, 4);               // WARNING: ignoring [[nodiscard]] return value

    int status = open_file("data.txt");  // OK: return value used
}