C++ Tutorial Index

C++ Tutorial C++ History C++ Installation C++ First Program C++ cin and cout C++ Data type C++ Variable C++ operator C++ Keywords

C++ Control Statements

C++ If C++ Nested if C++ If-else C++ If-else-if C++ Switch C++ Break C++ Continue C++ Goto C++ For loop C++ While loop C++ Do while loop

C++ Functions

C++ Call by Value C++ Call by Reference C++ Recursion Function C++ Inline function C++ Friend function

C++ Arrays

Single dimension array Two dimension array

C++ Strings

C++ Strings

C++ Inheritance

C++ Inheritance Single level Inheritance Multilevel Inheritance Multiple Inheritance Hierarchical Inheritance Hybrid Inheritance

C++ Polymorphism

C++ Polymorphism C++ Overloading C++ Overriding C++ Virtual Function

C++ Pointers

C++ Pointers C++ this pointer

C++ Exception Handling

C++ Exception Handling

C++ Constructors

C++ Constructors Default Constructor Parameterize Constructor Copy constructor Constructor Overloading Destructor

C++ File Handling

C++ File Handling C++ Writing to file C++ Reading file C++ Close file

Miscellaneous

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Passing by the pointer in C++ Free vs delete() in C++ goto statement in C and C++ C++ program to read string using cin.getline() C++ String Concatenation Heap Sort in C++ Swap numbers in C++ Input Iterators in C++ Fibonacci Series in C++ C ++ Program: Alphabet Triangle and Number Triangle C++ Program: Matrix Multiplication C++ Program to Print Fibonacci Triangle Stack in C++ Maps in C++ Queue in C++ C++ Bitset C++ Algorithms Priority Queue in C++ C++ Multimap C++ Deque Function Pointer in C++ Sizeof() Operators in C++ C++ array of Pointers free() Vs delete in C Timsort Implementation Using C++ CPP Templates C++ Aggregation C++ Enumeration C++ Math Functions C++ Object Class C++ Queue Initialize Vector in C++ Vector in C++ C++ STL Components Function overloading in C++ C++ Maximum Index Problem C++ find missing in the second array C++ Program to find the product array puzzle C++ Program To Find Largest Subarray With 0 Sum C++ Program To Move All Zeros To The End Of The Array C++ Program 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Programs to Print Pyramid Patterns in C++ swap() function in C++ Structure of C++ Program Stringstream in C++ and its applications rand() and srand() in C / C++ C++ Ternary Operator C++ Scope of Variables While Loop Examples in C++ Star pattern in C++ using For Loops For Loop Examples in C++ Do-While Loop Examples in C++ Top 5 IDEs for C++ That You Should Try Once Assertions in C/C++ C++ Convert Int to String Continue in C++ While loop Diamond Pattern in C++ using For Loop How to Reverse a String in C++ using Do-While Loop How to Reverse a String in C++ using For Loop How to Reverse a String in C++ using While Loop Infinite loop in C++ Loops in C++ Returning Multiple Values from a Function using Tuple and Pair in C++ wcscpy(), wcslen(), wcscmp() Functions in C++ Auto keyword in C++ C++ 11 vs C++ 14 vs C++ 17 C++ STL (Standard Template Library) Differences Between C Structures and C++ Structures Divide by Zero Exception in C++ Dynamic Constructor in C++ Dynamic Memory Allocation in C++ Find the Size of Array in C/C++ without using sizeof() function Floating Point Operations and Associativity in C, C++ and Java Hello World Program in C++ How to create a table in C++ How to Setup Environment for C++ Programming on Mac Implementation of a Falling Matrix in C++ Message Passing in C++ Pointer to Object in C++ Templates in C++ vs Generics in Java Ways to Copy a Vector in C++ What does Buffer Flush mean in C++ sort() function in C++ Structure Sorting (By Multiple Rules) in C++ Similarities between C++ and Java std::distance in C++ Array program in C++ C++ Tricks for Competitive Programming Desired Capabilities in Selenium Web Driver in C++ Socket Programming in C++ Template Specialization in C++ Classes and Objects in C++ Convex hull Algorithm in C++ DES in C++ C++ vardiac() function Difference between Two Sets in C++ Difference between Exit and Return Structured Binding in C++ Differences between Local and Global Variable Bitwise Operator vs Logical Operator Difference between OOP and POP in C++ Fork in C++ Functors in C++ How to call a void function in C++ How to create a directory or folder in C/C++ How to create a library in C++ How to create a stack in C++ How to create the Processes with Fork in C++ How to Handle Divide by Zero Exception in C++ Lambda Expression in C++ Pattern programs in C++ Roadmap to C++ Programming Substring in C++ Virtual base class in C++ Bits stdc++.h in C++ Top 14 Best Free C++ IDE (Editor & Compiler) for Windows in 2022 Bitmasking in C++ Auto Keyword in C++ Features of OOPS in C++ Hospital Management Project in C++ How to Declare Unordered Sets in C++ How to Sort an Array in C++ Include Guards in C++ Iostream in C++ Method overriding in C++ How to run program in turbo c++ How to Use Getline in C++ Leap Year Program in C++ Naming Convention in C++ New Operator in C++ Nullptr in C++ Object Slicing in C++ Principles of Object-Oriented Programming in C++ Processing strings using std string stream in C++ Pure Virtual 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in C/C++ Depth First Search Program to Traverse a Graph in C++ Features and Use Of Pointers in C/C++ Fread Function in C++ Programming Fscanf Function in The C++ Functions in C++ With Types and Examples Gmtime Function in C/C++ How is Multiset Implemented in C++ How to Build a Program in C++ How to Declare a 2d Array Dynamically in C++ inheritance Program in C++ int Max and int Min in C/C++ is It Fine to Write Void Main Or Main in C/C++ How to create a button in C++ abs() function in C++ Compile Time Polymorphism in C++ Division in C++ Factorial of a Number in C++ using while Loop Multiset in C++ 4 Pillars of OOPs Approach in C++ Backtracking Time Complexity in C++ C++ Global Variable C++ Pipe Tutorial Observer Design Pattern in C++ Private Inheritance in C++ Pthread in C++ Parameters SDL library in C++ with Examples Pointers in C++ Abstract Factory Design Pattern in C++ Ascending order in C++ How the value is passed in C++ Call by Pointer in C++ Constexpr in C++ Deadlock in C++ Design 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How to use the string find() in C++ Dynamic Casting in C++ 2D Vector Initialization in C++ C++ GUI Visual Studio C++ IPC C++ Macro Function Example C++ Permutation Overloading Stream Insertion in C++ Overloading array Index operator in C++ Operators that cannot be overloaded in C++ Operator overloading in C++ isprint() function in c++ Is_trivial function in C++ Is assignment operator Inherited in C++ div() function in C++ Default Assignment Operator and References in C++ Copy Constructor vs Assignment Operator in C++ Conversion Operator in C++ Array sum in C++ STL C++ Define Macro C++ Design C++ Factory Pattern TCP Client Server Example in C++ Convert String to Uppercase in C++ exit() and _Exit() in C and C++ Initializer list in C++ Iterator invalidation in C++ Lower bound in C++ Modulus of Two float numbers or double number Pass by value in C++ Set insert function in C++ Std partition_point in C++ Unary Operator Overloading in C++ Using Default Arguments with Virtual Functions Virtual 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For Auto in C++

Introduction

The original and updated meanings of this keyword are specified in the C++ standard. Prior to Visual Studio 2010, the "auto" keyword declared a local lifetime variable that is in the automatic storage class. The auto keyword defines a variable whose type is determined by the initialization statement in its declaration as of Visual Studio 2010 onward. The auto keyword's meaning is controlled by the /Zc:auto[-] compiler option.

Syntax:

auto declarator initializer ;
[](auto parameter1 , auto parameter2 ) {};

Remarks:

The “auto” keyword instructs the compiler to determine the type of a lambda expression parameter or declared variable from its initialization expression.

Unless you truly desire a conversion, we advise using the auto keyword because it offers the following advantages:

  • Robustness: It simply functions even when the type of the expression changes, including when the type of a function returns changes.
  • Performance: There won't be any conversion, you can count on it.
  • Usability: You do not have to be concerned about typos or name-spelling errors.
  • Efficiency: There is room for improvement in your coding.

Cases of conversion where using auto might not be a good idea include:

  • Nothing less than the desired type will do.
  • Using the auxiliary types for expressions, for instance, (valarray+valarray).

When declaring a variable, the auto keyword should be used in place of the type and an initialization phrase. Additionally, specifiers and declarators like pointer (*), const, reference (&), rvalue reference (&&) and volatile can be used to change the auto keyword (&&). The initialization expression is evaluated by the compiler, which then makes use of the results to determine the type of the variable.

There are various variations of the auto initialization expression:

  • The syntax for universal initialization, such as auto a { 42 };
  • the syntax for assignments, like auto x = 0;
  • The two preceding versions are combined in universal assignment syntax, such as auto c = 3.14159;.
  • Initialization is done directly or using constructor-style syntax, like auto d(1.41421f);.

A distinct initialization syntax is used when the auto is utilized to define the loop parameter in a range based on a statement, such as for (auto& I iterable) do action(i);.

The auto keyword serves as a stand-in for a type but is not a type in and of itself. As a result, the auto keyword cannot be used with operators like sizeof or (for C++/CLI) typeid or with casts.

Usefulness:

A straightforward method to define a variable with a complex type is to use the auto keyword. When the initialization statement contains pointers to functions, templates, or pointers to members, for instance, you can define a variable using auto.

A variable can be declared and initialized to a lambda expression using auto as well. Since only the compiler is aware of the type of a lambda expression, you cannot define the type of the variable yourself. 

Types of Trailing Returns

To assist in creating template libraries, utilize auto and the decltype type specifier. Declare a function template whose return type depends on the types of its template arguments using auto and decltype. Alternately, you can declare a function template using auto and decltype that wraps a call to some other function and returns whatever that other function's return type is.

CV Qualifications and References

Utilizing volatile qualifiers, const qualifiers, and auto drops references. Think about the following instance:

Code

#include <iostream>
using namespace std;


int main( )
{
    int c = 10;
    int&count_Ref = c;
    auto my_Auto = count_Ref;


count_Ref = 11;
cout<< c << " ";


my_Auto = 12;
cout<< c <<endl;
}

Output:

[Running] cd "d:\Programming\C++\" && g++ test.cpp -o test && "d:\Programming\C++\"test
11 11


[Done] exited with code=0 in 0.773 seconds

Because my_Auto is an "int", not an int reference, the output in the previous example is 11 11, rather than 11 12, as it would have been if auto hadn't lost the reference qualifier.

Braced Initializers and Type Deduction (C++14)

Braces are used to initialize an auto variable in the following piece of code. Keep in mind the difference between b and c as well as a and e.

Code:

#include <initializer_list>


int main()
{
    auto a = { 2, 3 }; // std::initializer_list<int>


    auto b = { 4 }; // std::initializer_list<int>


    auto c{ 5 };  // int


    // C3535: 'auto' type cannot be determined from the initializer list'
    auto d = { 6, 7.7 };


    // The type for "auto" in C3518 can only be determined from a single initializer phrase in a context including direct-list initialization.
    auto e{ 9, 10 };


    return 0;
}

Output:

[Running] cd "d:\Programming\C++\" && g++ tempCodeRunnerFile.cpp -o tempCodeRunnerFile&& "d:\Programming\C++\"tempCodeRunnerFile
tempCodeRunnerFile.cpp: In function 'int main()':
tempCodeRunnerFile.cpp:12:23: error: unable to deduce 'std::initializer_list<auto>' from '{6, 7.7000000000000002e+0}'
     auto d = { 6, 7.7 };
                       ^
tempCodeRunnerFile.cpp:12:23: note:   deduced conflicting types for parameter 'auto' ('int' and 'double')
tempCodeRunnerFile.cpp:15:19: error: direct-list-initialization of 'auto' requires exactly one element [-fpermissive]
     auto e{ 9, 10 };
                   ^
tempCodeRunnerFile.cpp:15:19: note: for deduction to 'std::initializer_list', use copy-list-initialization (i.e. add '=' before the '{')


[Done] exited with code=1 in 0.157 seconds

Error Messages and Restrictions

The following table details the limitations on the usage of the auto keyword as well as the diagnostic error message the compiler generates when those limitations are met.

Error numberDescription
C3530Any other type of specifier cannot be used with the “auto” keyword.
C3531An initializer is required for symbols that are specified using the “auto” keyword.
C3532You erroneously declared a type using the “auto” keyword. You might have declared an array or a method return type, for instance.
C3533, C3539The “auto” keyword cannot be used to declare either a parameter or a template argument.
C3534The “auto” keyword cannot be used to declare a method or template parameter.
C3535A symbol must first be initialized before it can be utilized. The practical implication is that a variable cannot be used to initialize itself.
C3536A type that is specified with the “auto” keyword cannot be cast.
C3538A declarator list with the “auto” keyword defined must resolve each symbol to the same type. 
C3540, C3541A symbol that has the “auto” keyword in its declaration cannot be subjected to the "sizeof" and "typeid" operators.

Examples:

These bits of code serve as examples of several possible uses for the auto keyword.

The declarations that follow are interchangeable. The type of variable i is specified to be int in the first sentence. Because initialization expression (0) is an integer, it can be assumed that variable j in the second sentence is of type int.

Code:

// The explicit type of variable i is int.
int i = 0;  
// Because zero is an integer, the variable jhas the implicit type of int.
auto j = 0;

The declarations that follow are identical, but the second one is easier to understand. The convenience of using the “auto” keyword is among its most appealing arguments.

Code:

map<int,list<string>>::iterator k = m.begin();
auto k = m.begin();

When the "for" and range for loops begin, the following code piece declares the types of the variables itr and elm.

Code:

#include <deque>
using namespace std;


int main()
{
    deque<double>dq_Double_Data(10, 0.1);


    for (auto itr = dq_Double_Data.begin(); itr != dq_Double_Data.end(); ++itr)
    { /* ... */ }


    // Keeping the following information in mind, choose range-for loops
    //(Any range-for with auto is affected by this, not just deque)




    // COPIES elements, which is only somewhat better than the other examples.
    for (auto elm :dq_Double_Data) 
    { /* ... */ }


    // observes elements IN-PLACE and/or adjusts them
    for (auto&elm :dq_Double_Data) 
    { /* ... */ }


    // examines elements IN-PLACE
    for (const auto&elm :dq_Double_Data) 
    { /* ... */ }
}

The “new” operator and a pointer declaration are used to declare pointers in the following code segment.

Code:

double a = 12.34;
auto *b = new auto(a), **c = new auto(&b);

The following snippet of code includes several symbol declarations in every declaration statement. Keep in mind that each statement's symbols are all resolved to the same type.

Code:

// Resolves to int.
auto a = 1, *b = &a, **c = &b; 
// Resolves to double.
auto x(2.01), *y (&a);   
// Resolves to char.      
auto z = 'x', *p(&z);    
// Resolves to int.      
auto n = 1, &m = n;     

The conditional operator (?:) is used in the following code fragment to declare variable x as an integer with the value 300:

Code:

int v_1 = 100, v_2 = 300;
auto x = v_1 > v_2 ? v_1 : v_2;

The code snippet that follows initializes the variables a to be of type int, b to be a reference to a variable of type const int, and fun_p to be a pointer to the function that returns a variable of type int.

Code:

int fun(int a) 
{ return a; }
int main()
{
    auto a = fun(0);
    const auto& b = fun(1);
    int (*ptr)(int a);
ptr = fun;
    auto fun_p = ptr;
}