Lambda Expressions

Map of STL Algorithms

Lambda Expressions

Map of STL Algorithms

The World Map of C++ STL Algorithms

List of algorithms

Lambda

Lambda expression creates anonymous function object

[capture-list] (parameter-list) {statement-list}

Minimal lambda: [] {}

With return type:

[capture-list] (parameter-list) -> return-type {statement-list}

Lambdas without capture (stateless) are compatible with plain C functions

auto lambda = [](int i) -> int { return 42*i; } // precise type

using func_t = int(*)(int);         // type of function-pointer
typedef int (*func_t)(int);         // C version using typedef
func_t p_lambda = lambda;           // type decay from lambda to function pointer

• Default capture by reference may lead to dangling references to destroyed scope.
• Default capture by values may lead to dangling pointers to destroyed scope (through this).
• Default capture by values does not capture static variables, but uses them by reference instead.
• Default capture is safe when used locally, but even then someone may copy-paste them into unsafe environment
• $\Rightarrow$ explicit capture forces to think, hence safer

Lambda Init

• Capture ownership (unique_ptr, future, thread)
• [ var = initExpr ] (...) {...}
• Left of = is inner scope, right of = is outer scope

class Widget {
 bool isValidated() const;
 bool isArchived() const;
};
auto pw     = std::make_unique<Widget>();
auto test = [pw = std::move(pw)] // reuse the "pw" name
 { return pw->isValidated() && pw->isArchived(); }

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Last update: June 15, 2021