Q: cpp lanbda expression
auto val = []() -> <type>{
};
I can provide variables inside [] n () also. But what is difference?
chatGpt:
In C++, lambda expressions can capture variables in two ways: through the capture list (`[]`) and the parameter list (`()`). Understanding the difference between these two is crucial for effectively using lambdas in C++.
### 1. **Capture List (`[]`)**
The capture list is used to capture variables from the surrounding scope (i.e., the scope in which the lambda is defined). The captured variables can be accessed inside the lambda's body.
- **Syntax:** `[]`
- **Purpose:** To specify which variables from the enclosing scope are accessible inside the lambda.
#### Types of Captures:
- **Capture by Value:** `[x]` captures the variable `x` by value. The lambda will create a copy of `x`.
- **Capture by Reference:** `[&x]` captures the variable `x` by reference. The lambda will use the original `x`.
- **Capture All by Value:** `[=]` captures all variables used inside the lambda by value.
- **Capture All by Reference:** `[&]` captures all variables used inside the lambda by reference.
#### Example:
```cpp
int a = 10, b = 20;
auto lambda = [a, &b]() {
// a is captured by value, b is captured by reference
std::cout << "a: " << a << ", b: " << b << std::endl;
};
lambda();
b = 30;
lambda(); // Note that a remains 10, while b changes to 30
```
### 2. **Parameter List (`()`)**
The parameter list is used to pass arguments to the lambda function, just like in a regular function. These are not captured variables from the enclosing scope but rather arguments passed when the lambda is invoked.
- **Syntax:** `()`
- **Purpose:** To define parameters that the lambda can accept when it is called.
#### Example:
```cpp
auto lambda = [](int x, int y) -> int {
return x + y;
};
int result = lambda(5, 10); // Passes 5 and 10 as arguments
```
### **Differences:**
- **Scope of Variables:**
- **Capture List (`[]`)**: Captures variables from the surrounding scope of where the lambda is defined.
- **Parameter List (`()`)**: Accepts variables as input parameters when the lambda is invoked.
- **When to Use:**
- **Capture List (`[]`)**: Use this when you need to access or modify variables from the enclosing scope inside the lambda.
- **Parameter List (`()`)**: Use this when the lambda needs to take arguments at the time of invocation, similar to a regular function.
### Combined Example:
```cpp
int a = 5;
auto lambda = [a](int x) -> int {
return a + x; // 'a' is captured from the surrounding scope, 'x' is passed as an argument
};
int result = lambda(10); // result is 15 (5 from 'a' and 10 from 'x')
```
### Summary:
- **Capture List (`[]`)**: Captures variables from the surrounding context for use within the lambda.
- **Parameter List (`()`)**: Defines parameters that must be provided when calling the lambda.
Both the capture list and parameter list can be used together to create powerful and flexible lambda expressions in C++.
Q:
[=] captures all variables used inside the lambda by value.[&] captures all variables used inside the lambda by reference.