7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.5 Lambda expressions [expr.prim.lambda] General [expr.prim.lambda.general]

A lambda-expression provides a concise way to create a simple function object.
[Example 1: #include <algorithm> #include <cmath> void abssort(float* x, unsigned N) { std::sort(x, x + N, [](float a, float b) { return std::abs(a) < std::abs(b); }); } — end example]
A lambda-expression is a prvalue whose result object is called the closure object.
[Note 1: 
A closure object behaves like a function object.
— end note]
An ambiguity can arise because a requires-clause can end in an attribute-specifier-seq, which collides with the attribute-specifier-seq in lambda-expression.
In such cases, any attributes are treated as attribute-specifier-seq in lambda-expression.
[Note 2: 
Such ambiguous cases cannot have valid semantics because the constraint expression would not have type bool.
— end note]
A lambda-specifier-seq shall contain at most one of each lambda-specifier and shall not contain both constexpr and consteval.
If the lambda-declarator contains an explicit object parameter ([dcl.fct]), then no lambda-specifier in the lambda-specifier-seq shall be mutable or static.
The lambda-specifier-seq shall not contain both mutable and static.
If the lambda-specifier-seq contains static, there shall be no lambda-capture.
[Note 3: 
The trailing requires-clause is described in [dcl.decl].
— end note]
If a lambda-declarator does not include a parameter-declaration-clause, it is as if () were inserted at the start of the lambda-declarator.
If the lambda-declarator does not include a trailing-return-type, it is considered to be -> auto.
[Note 4: 
In that case, the return type is deduced from return statements as described in [dcl.spec.auto].
— end note]
[Example 2: auto x1 = [](int i) { return i; }; // OK, return type is int auto x2 = []{ return { 1, 2 }; }; // error: deducing return type from braced-init-list int j; auto x3 = [&]()->auto&& { return j; }; // OK, return type is int& — end example]
A lambda is a generic lambda if the lambda-expression has any generic parameter type placeholders ([dcl.spec.auto]), or if the lambda has a template-parameter-list.
[Example 3: int i = [](int i, auto a) { return i; }(3, 4); // OK, a generic lambda int j = []<class T>(T t, int i) { return i; }(3, 4); // OK, a generic lambda — end example]