1. A brief look at some of the new features introduced to the language
C++ ’11 and C++ ‘14
A brief look at some of the new features introduced to the language
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2. Copyright © 2012-2017 by Curt Hill
History
Although the language was developed at Bell Labs, it has periodically gone through the standardization process
There was one in 1985, 1998, 2003 and the most recent was in 2011
Bjarne Stroustrup has led the team in 1979 at Bell Labs and has always been seriously involved
The standard has sometimes been referred to c++’0x
They anticipated earlier finalization
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3. Standards and Compilers
Some compilers take more pride in adhering to the standard
Typically the open source come the closest, but slowest
GCC
Commercial compilers want to be seen as modern so tend to comply
However, they want to be non-standard to keep their users from migrating
Some features have already been implemented some are waiting
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4. Copyright © 2012-2017 by Curt Hill
Lambda Expressions
A mathematical expression devised in 20th century
These have been in LISP since 1959
In Java as well
A lambda expression is a function that is defined right where needed
Instead of defined before and called
Also known as anonymous functions
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5. Functions as Parameters
For some time C++ has supported function objects
Really just a pointer at a function
Recall the linked list example
We would like to apply a function to each item in the list
There are two ways:
Iterators
Functions as parameter
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Example
class LinkedNode{
private:
friend LinkedList;
float data;
LinkedNode * next;
LinkedNode(); };
class LinkedList{
friend class LinkedIterator;
LinkedNode * root
public:
void touchAll(void (*fn)(float));
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What was that?
Did you notice: void touchAll(void (*fn)(float));
It is a function that takes a function as a parameter
A typical definition follows
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Example
void LinkedList::touchAll(
void (*fn)(float)){
LinkedNode * ptr = root;
while(ptr != null){
(*fn)(ptr->data);
ptr = ptr -> next; }
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Continuing
I may now pass any function that has the correct signature to touchAll
I mention the function, but I give no parameters to it
Consider the next screen
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Example
void sum(float f){…}
void max(float f){…}
void min(float f){…}
LinkedList myList; …
myList.touchAll(sum);
Mylist.touchAll(max);
Mylist.touchAll(min);
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Commentary
Any function that takes a float and has return type void may be passed to touchAll
The method touchAll will walk through the list
At each node it will call the passed function
Without ever knowing what the function actually was or did
That is a function as a parameter or a function object
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12. Copyright © 2012-2017 by Curt Hill
Lambdas
The form is: [capture](params)->return-type {body}
The capture part is in square brackets and describes the local variables that may be used
These may be by value or reference
Params are like any parameter
Return type is optional if all the returns agree in type
The body looks like any function’s body
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Example
A simple example: [](int x, int y) { return x + y; }
This could only be used where a two integer function could be used
Recall the touchAll method
Now we no longer need a defined function we can just make up one as we go along
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Example
void doIt(LinkedList & mL){
float sum = 0; …
ml.touchAll([&sum](float f){
sum += f;
} );
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What was that?
The touchAll method expects a function
It gets one made up on the spot
The capture list allowed the anonymous function to access sum by reference
This saved the sum in a local variable of the function doIt
There are many times we can pass functions, now they do not have to already defined
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16. Generalized Constants
Recall that an array must have a constant size
Provided it is not dynamic
Thus the following is not legal int get_five() {return 5;} int array [get_five()+7];
Whereas: int array [5+7];
Is legal
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Commentary
It is clear to us that this should be OK, but the compiler is not allowed to peek inside the get_five and determine that it has no run-time effects
One of the problems is that when the compiler looks at get_five it does not know what it will be used for
Thus it cannot allow this
The 2011 standard introduces a constexpr prefix and keyword that allows this
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New Array Example
Now we can say: constexpr int get_five() {return 5;} int array [get_five()+7];
This is legal
If a function with the constexpr prefix does anything that may only be done at runtime a syntax error is issued
Similar to an attempt to change a constant reference parameter
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Type Inference
In previous versions every data value must have an explicitly declared type
Variables, constants, parameters, function results
The language will now let the type be left out if it may be inferred
This usually requires the auto keyword auto myVar = 10;
The integer type may be inferred
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Expansion
In C this is expanded
The auto may be the return type in a function
However you must be consistent: auto fn(int a){ if(a>5) return 0; return 5.5; }
This is not legal since two types are returned
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Another
Also in 2014 a lambda function may have an auto parameter
This makes it basically a generic
Consider: auto adder = [] (auto op1, auto op2){ return op1+op2; }
This can be used multiple times with different parameter types
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Templates
Recall the problems with templatizing the set program?
In the client specific types had to be used
The template class needed no modification, but the client did
With this we can generalize the client in the same way that we generalized the list
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Range Based For
Often we use a for to process a range that we could infer
Similar to what is done in Java
Consider: int arr[10]; … for(int i = 0;i<10;i++)
There is now a for that will infer the range: for(int &x: arr) x++;
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For Usage
Similar to a construct in Java
Using: for(int x: ob)
It will assign to x each successive element of ob
If x is preceded by & the element may be changed
Otherwise it is a read only access
This may be used in any way the subscripted variable may be
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Range For Data
It may be used for:
Arrays whose declaration may be seen
Not those passed as parameters
Initializer lists
Enumerations
for(int x:{2,4,9,16}) …
Any object that has a begin() and end() method
Such as STL iterators
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Deprecated
The 2014 standard also adds a deprecated attribute
Implementation in a compiler is optional but a warning on using it is generally advisable
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Conclusion
There are several other things too numerous to mention here
These are available in several compilers
The 2017 standard will soon make changes
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