Functions, Methods, and Lambdas
PB173 Programming in Modern C++
Nikola Beneš, Vladimír Štill, Jiří Weiser
Faculty of Informatics, Masaryk University
spring 2016
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 1 / 19
Overview
function as a parameter
method as a parameter
lambda
definition
capture list
as a parameter
C++ libraries
algorithm library
iterator library
lazy library
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 2 / 19
Overview
function as a parameter
method as a parameter
lambda
definition
capture list
as a parameter
C++ libraries
algorithm library
iterator library
lazy library
. . . oh wait, that was the homework
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 2 / 19
Motivation
Why we do want it?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 3 / 19
Motivation
Why we do want it?
Cause it is cool!
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 3 / 19
Motivation
Why we do want it?
Cause it is cool!
Do we need it?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 3 / 19
Motivation
Why we do want it?
Cause it is cool!
Do we need it?
No, but C++ without lambdas as a concept would be like
Java without classes or Haskell without functions.
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 3 / 19
Function as a parameter
int foo( int a, int b ) {
return a * 3 + b;
}
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 4 / 19
Function as a parameter
int foo( int a, int b ) {
return a * 3 + b;
}
How to create a pointer to function?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 4 / 19
Function as a parameter
int foo( int a, int b ) {
return a * 3 + b;
}
How to create a pointer to function?
int main() {
auto f = foo;
std::cout << f( 3, 8 ) << std::endl;
}
What is the type of f?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 4 / 19
Function as a parameter
int foo( int a, int b ) {
return a * 3 + b;
}
The type of the foo function is int( int, int )
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 5 / 19
Function as a parameter
int foo( int a, int b ) {
return a * 3 + b;
}
The type of the foo function is int( int, int )
using FooType = int( int, int );
// FooType is NOT a pointer
FooType *ptrToFoo = foo;
int(*ptrToFoo2)(int, int) = foo;
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 5 / 19
Function as a parameter
There was a question:
Why do function pointer definitions work with any number
of ampersands ‘&’ or asterisks ’*’?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 6 / 19
Function as a parameter
There was a question:
Why do function pointer definitions work with any number
of ampersands ‘&’ or asterisks ’*’?
void foo() { cout << "Foo to you too!\n"; }
int main() {
void (*p1)() = foo; void (*p2)() = *foo;
void (*p3)() = &foo; void (*p4)() = *&foo;
void (*p5)() = &*foo;void (*p6)() = **foo;
void (*p7)() = **********************foo;
(*p1)(); (*p2)(); (*p3)();
(*p4)(); (*p5)(); (*p6)();
(*p7)();
}
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 6 / 19
Function as a parameter
Explanation
expression foo is implicitly convertible to a pointer to the
function
expression *foo results to foo
expression &foo takes an address of the function (i.e. a pointer
to the function)
expressions can be combined together
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 7 / 19
Method as a parameter
struct X {
int foo( int a, int b ) {
return a * 3 + b;
}
};
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 8 / 19
Method as a parameter
struct X {
int foo( int a, int b ) {
return a * 3 + b;
}
};
How to create a pointer to a member function?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 8 / 19
Method as a parameter
struct X {
int foo( int a, int b ) {
return a * 3 + b;
}
};
How to create a pointer to a member function?
int main() {
X x;
auto f = &X::foo;
}
What is the type of f?
How can we call f?
Is the ampersand necessary?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 8 / 19
Method as a parameter
What is the type of f?
int (X::*)( int, int )
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 9 / 19
Method as a parameter
What is the type of f?
int (X::*)( int, int )
How can we call f?
(x.*f)( 3, 8 )
(ptrToX->*f)( 3, 8 )
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 9 / 19
Method as a parameter
What is the type of f?
int (X::*)( int, int )
How can we call f?
(x.*f)( 3, 8 )
(ptrToX->*f)( 3, 8 )
Is the ampersand necessary?
Yes. Rules for taking address of member function are different
to the old C rules for plain functions.
No. Just for Visual Studio.
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 9 / 19
Lambda – definition
[capture list] ( parameters ) -> return_type { body }
capture list
which variables from outside should be visible in the lambda
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 10 / 19
Lambda – definition
[capture list] ( parameters ) -> return_type { body }
capture list
which variables from outside should be visible in the lambda
parameters
the same list as in the functions
. . . or auto as a type
can be ommited if the list is empty
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 10 / 19
Lambda – definition
[capture list] ( parameters ) -> return_type { body }
capture list
which variables from outside should be visible in the lambda
parameters
the same list as in the functions
. . . or auto as a type
can be ommited if the list is empty
return type
it is what it represents. . .
can be ommited if the type is obvious
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 10 / 19
Lambda – definition
[capture list] ( parameters ) -> return_type { body }
capture list
which variables from outside should be visible in the lambda
parameters
the same list as in the functions
. . . or auto as a type
can be ommited if the list is empty
return type
it is what it represents. . .
can be ommited if the type is obvious
body
the code
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 10 / 19
Lambda – capture list
list of variables
with ampersand – references
without ampersand – copies
const
this
& – capture all as a reference
= – capture all by copy
const
initializer
introducing new variable
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 11 / 19
Lambda – capture list
Examples
[=,this,&events] {
++events;
return this->foo( a ) + b;
}
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 12 / 19
Lambda – capture list
Examples
[=,this,&events] {
++events;
return this->foo( a ) + b;
}
int x = 4;
int y = [&r = x, x = x + 1] {
++r;
return x + 1;
}();
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 12 / 19
Lambda as a parameter
What is the type of lambda?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 13 / 19
Lambda as a parameter
What is the type of lambda?
Who knows. . . (The compiler knows.)
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 13 / 19
Lambda as a parameter
What is the type of lambda?
Who knows. . . (The compiler knows.)
How to store lambdas?
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 13 / 19
Lambda as a parameter
What is the type of lambda?
Who knows. . . (The compiler knows.)
How to store lambdas?
auto
template parameter
std::function< signature >
runtime overhead
pointer to function
requires empty capture list
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 13 / 19
Lambda as a parameter
Example
void e1( void(*f)( int ), int p ) { f( p ); }
template< typename F >
void e2( F f, int p ) { f( p ); }
void e3( std::fucntion< void(int) > f, int p ) {
f( p );
}
auto foo = []( int i ) { std::cout << i; };
e1( foo, 1 );
e2( foo, 1 );
e3( foo, 1 );
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 14 / 19
How to use lambdas
Do not overuse them.
Lambdas should be short. Four lines at maximum.
If you need to name the lambda, use a function instead.
If your capture list is long, choose a different approach.
If your lambda is long, use a function or a method instead.
Prefer references to copies in the capture list.
The generated class will be smaller.
References could be dangerous. Be careful.
If the number of lambdas is higher than number of methods,
you should consider refactoring your code.
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 15 / 19
C++ libraries
algorithm library
copy, transform, generate
remove, reverse, fill
equals, find, count
iterator library
back_inserter
istream_iterator
ostream_iterator
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 16 / 19
Exercise 1
Implement template function forEach so that:
takes two input iterators
first and last
takes a function callback as a third parameter
by pointer to function
by template parameter
by std::function
use a simple lambda
increment parameter, multiply parameter, . . .
template< typename It, /*...*/ >
void forEach( It first, It last, /*...*/ f ) { /*...*/ }
Compare the speed on large container.
Use SequenceGenerator from study materials.
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 17 / 19
Exercise 2
Refactor 03_lines.cpp so that:
reading of lines is placed in a new function
printing is realized by a lambda function
template< typename F >
void readLines( const char *file, F f ) {
// ...
}
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 18 / 19
Exercise 2
Refactor 03_lines.cpp so that:
reading of lines is placed in a new function
printing is realized by a lambda function
template< typename F >
void readLines( const char *file, F f ) {
// ...
}
change behaviour to print only even lines
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 18 / 19
Exercise 3
Refactor 03_algorithm.cpp so that:
no explicitly written cycle is present
function almostSame works for all containers
not just those with the random access
use constructs from
algorithm library
iterator library
PB173 Modern C++: Functions, Methods, and Lambdas spring 2016 19 / 19