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c++optional-arguments
Swapnil Gupta
Swapnil Gupta

Reputation: 8941

How to pass optional arguments to a method in C++?

How to pass optional arguments to a method in C++ ? Any code snippet...

Upvotes: 170

Views: 291397

Answers (9)

KenJ
KenJ

Reputation: 637

To follow the example given here, but to clarify syntax with the use of header files, the function forward declaration contains the optional argument default value.

myfile.h

void myfunc(int blah, int mode = 0);

myfile.cpp

void myfunc(int blah, int mode) /* mode = 0 */
{
    if (mode == 0)
        do_something();
     else
        do_something_else();
}

Upvotes: 52

fad
fad

Reputation: 53

Jus adding to accepted ans of @Pramendra , If you have declaration and definition of function, only in declaration the default param need to be specified

Upvotes: 0

Fernando G. Testa
Fernando G. Testa

Reputation: 496

With the introduction of std::optional in C++17 you can pass optional arguments:

#include <iostream>
#include <string>
#include <optional>

void myfunc(const std::string& id, const std::optional<std::string>& param = std::nullopt)
{
    std::cout << "id=" << id << ", param=";

    if (param)
        std::cout << *param << std::endl;
    else
        std::cout << "<parameter not set>" << std::endl;
}

int main() 
{
    myfunc("first");
    myfunc("second" , "something");
}

Output:

id=first param=<parameter not set>
id=second param=something

See https://en.cppreference.com/w/cpp/utility/optional

Upvotes: 34

Pramendra Gupta
Pramendra Gupta

Reputation: 14873

Here is an example of passing mode as optional parameter 

void myfunc(int blah, int mode = 0)
{
    if (mode == 0)
        do_something();
     else
        do_something_else();
}

you can call myfunc in both ways and both are valid

myfunc(10);     // Mode will be set to default 0
myfunc(10, 1);  // Mode will be set to 1

Upvotes: 208

Alok Save
Alok Save

Reputation: 206636

An important rule with respect to default parameter usage:
Default parameters should be specified at right most end, once you specify a default value parameter you cannot specify non default parameter again. ex: 

int DoSomething(int x, int y = 10, int z) -----------> Not Allowed

int DoSomething(int x, int z, int y = 10) -----------> Allowed

Upvotes: 74

user2008151314
user2008151314

Reputation: 680

It might be interesting to some of you that in case of multiple default parameters:

void printValues(int x=10, int y=20, int z=30)
{
    std::cout << "Values: " << x << " " << y << " " << z << '\n';
}

Given the following function calls:

printValues(1, 2, 3);
printValues(1, 2);
printValues(1);
printValues();

The following output is produced:

Values: 1 2 3
Values: 1 2 30
Values: 1 20 30
Values: 10 20 30

Reference: http://www.learncpp.com/cpp-tutorial/77-default-parameters/

Upvotes: 43

Prasoon Saurav
Prasoon Saurav

Reputation: 92922

Use default parameters

template <typename T>
void func(T a, T b = T()) {

   std::cout << a << b;

}

int main()
{
    func(1,4); // a = 1, b = 4
    func(1);   // a = 1, b = 0

    std::string x = "Hello";
    std::string y = "World";

    func(x,y);  // a = "Hello", b ="World"
    func(x);    // a = "Hello", b = "" 

}

Note : The following are ill-formed

template <typename T>
void func(T a = T(), T b )

template <typename T>
void func(T a, T b = a )

Upvotes: 17

Jerry Coffin
Jerry Coffin

Reputation: 490673

Typically by setting a default value for a parameter:

int func(int a, int b = -1) { 
    std::cout << "a = " << a;
    if (b != -1)        
        std::cout << ", b = " << b;
    std::cout << "\n";
}

int main() { 
    func(1, 2);  // prints "a=1, b=2\n"
    func(3);     // prints "a=3\n"
    return 0;
}

Upvotes: 8

Ben Voigt
Ben Voigt

Reputation: 283893

With commas separating them, just like parameters without default values.

int func( int x = 0, int y = 0 );

func(); // doesn't pass optional parameters, defaults are used, x = 0 and y = 0

func(1, 2); // provides optional parameters, x = 1 and y = 2

Upvotes: 10

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