Zephyr's Notes on ISCS & CBMS, UTokyo

参数传递机制

Nov 16, 20246 min read

参数传递机制 | Parameter Passing Mechanisms

参数传递机制决定了在函数调用过程中，实参（实际参数）如何传递给形参（形式参数）。不同的编程语言采用不同的参数传递机制，这影响了函数调用的行为和性能。

Parameter passing mechanisms determine how actual parameters are passed to formal parameters during a function call. Different programming languages use different parameter passing mechanisms, which affect the behavior and performance of function calls.

常见参数传递机制 | Common Parameter Passing Mechanisms

1. 按值传递 (Call by Value)

定义 | Definition

按值传递机制中，实参的值在调用时被复制给形参。函数内部对形参的修改不会影响实参的值。

In the call by value mechanism, the value of the actual parameter is copied to the formal parameter at the time of the call. Modifications to the formal parameter within the function do not affect the actual parameter.

特点 | Characteristics

  • 安全性 | Safety

    • 实参的值不会被修改。
    • The value of the actual parameter is not modified.

  • 独立性 | Independence

    • 函数调用之间的参数相互独立。
    • Parameters are independent across function calls.

示例 | Example

def increment(x):
    x += 1
    return x
 
a = 5
print(increment(a))  # Output: 6
print(a)  # Output: 5

2. 按引用传递 (Call by Reference)

定义 | Definition

按引用传递机制中，实参的引用（地址）被传递给形参。函数内部对形参的修改会影响实参的值。

In the call by reference mechanism, the reference (address) of the actual parameter is passed to the formal parameter. Modifications to the formal parameter within the function affect the actual parameter.

特点 | Characteristics

  • 共享性 | Shared

    • 实参和形参共享相同的存储位置。
    • The actual and formal parameters share the same storage location.

  • 副作用 | Side Effects

    • 函数内部对形参的修改会影响实参。
    • Modifications to the formal parameter within the function affect the actual parameter.

示例 | Example

def increment(lst):
    lst[0] += 1
 
a = [5]
increment(a)
print(a)  # Output: [6]

3. 按值结果传递 (Call by Value-Result)

定义 | Definition

按值结果传递机制中，实参的值在调用时被复制给形参，函数返回时形参的值被复制回实参。也称为按拷贝传递（Call by Copy）。

In the call by value-result mechanism, the value of the actual parameter is copied to the formal parameter at the time of the call, and the value of the formal parameter is copied back to the actual parameter when the function returns. It is also known as call by copy.

特点 | Characteristics

  • 双向传递 | Bidirectional Transfer

    • 实参值被复制给形参，函数返回时形参值被复制回实参。
    • The actual parameter’s value is copied to the formal parameter, and the formal parameter’s value is copied back to the actual parameter upon function return.

  • 潜在的副作用 | Potential Side Effects

    • 函数内部对形参的修改会影响实参。
    • Modifications to the formal parameter within the function affect the actual parameter.

示例 | Example

def increment(x):
    x += 1
    return x
 
a = 5
a = increment(a)
print(a)  # Output: 6

4. 按名称传递 (Call by Name)

定义 | Definition

按名称传递机制中，实参在函数调用时不会立即求值，而是在每次使用形参时重新计算实参的表达式。

In the call by name mechanism, the actual parameter is not evaluated at the time of the call; instead, the expression representing the actual parameter is re-evaluated each time the formal parameter is used.

特点 | Characteristics

  • 惰性求值 | Lazy Evaluation

    • 实参表达式仅在需要时求值，可能导致多次求值。
    • The actual parameter expression is evaluated only when needed, potentially leading to multiple evaluations.

  • 灵活性 | Flexibility

    • 允许传递复杂的表达式，而不是简单的值。
    • Allows passing complex expressions rather than simple values.

示例 | Example

function f(x) {
    return x + x;
}
 
a = 3;
b = a + 2;
 
result = f(b);

按名称传递的结果为：

Call by name results in:

result = f(a + 2) = (a + 2) + (a + 2) = 5 + 5 = 10;

5. 按需传递 (Call by Need)

定义 | Definition

按需传递是按名称传递的惰性求值版本，实参表达式在第一次使用时求值，并在后续使用中缓存结果。也称为惰性传递（Lazy Evaluation）。

Call by need is a lazy evaluation version of call by name, where the actual parameter expression is evaluated the first time it is used and the result is cached for subsequent uses. It is also known as lazy evaluation.

特点 | Characteristics

  • 惰性求值 | Lazy Evaluation

    • 实参表达式仅在第一次使用时求值，并缓存结果。
    • The actual parameter expression is evaluated only the first time it is used and the result is cached.

  • 避免重复计算 | Avoids Repeated Evaluation

    • 避免了按名称传递的多次求值问题。
    • Avoids the repeated evaluation problem of call by name.

示例 | Example

-- Haskell example of call by need (lazy evaluation)
f :: Int -> Int
f x = x + x
 
a = 3
b = a + 2
 
result = f b  -- Result is 10

总结 | Summary

不同的参数传递机制在函数调用中有不同的行为和应用场景。理解这些机制及其特点有助于选择合适的编程语言和编写高效的代码。

Different parameter passing mechanisms exhibit different behaviors and use cases in function calls. Understanding these mechanisms and their characteristics helps in choosing the appropriate programming language and writing efficient code.

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