Set default parameter value from another paramater's value [duplicate]

Question

Is it possible to achieve such functionality, that parameter's value would default to another parameter's value if not specified?

Example:

class Health
{
public:
    // If current is not specified, its value defaults to max's value
    Health(int max, int current = max) : max_(max), current_(current) { }
    int max_;
    int current_;
};

As it is now, I am getting a compile error:

error: 'max' was not declared in this scope
Health(int max, int current = max) : max_(max), current_(current) { }
                              ^

Jarod42 · Accepted Answer · 2015-08-12 14:38:12Z

8

You have to provide overload:

class Health
{
public:
    Health(int max, int current) : max_(max), current_(current) { }

    Health(int max) : max_(max), current_(max) {}
    // or `Health(int max) : Health(max, max) {}` since C++11

    int max_;
    int current_;
};

answered Aug 12, 2015 at 14:38

Jarod42

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Comments

NathanOliver · Accepted Answer · 2015-08-12 14:42:42Z

0

You can default the parameter to a value that should not be accepted and then you and then check using the ? operator in the initialization

class Health
{
public:
    // If current is not specified, its value defaults to max's value
    Health(int max, int current = 0) : max_(max), current_(current == 0 ? max : current) { }
    int max_;
    int current_;
};

edited Aug 12, 2015 at 14:42

answered Aug 12, 2015 at 14:40

NathanOliver

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3 Comments

Borgleader Over a year ago

What if 0 is a valid value for current?

Jarod42 Over a year ago

or use const int* current=nullptr (or optional<int>), but it seems over complicated.

Angew is no longer proud of SO Over a year ago

@Jarod42 The pointer would be super-ugly for the caller ("what do you mean I cannot pass a literal?!"), but optional is fair game, I'd say.

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