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A member function declared inside a C++ class is private by default. Inside a struct or union, it is public by default. This rule applies to member functions—not to every function in a C++ program.

How the default works

The default access applies to member declarations in the type definition. Member functions follow the same rule as data members and other members.

class Example {
    void hidden();       // private by default
public:
    void visible();      // public after the access label
};

struct Record {
    void visible();      // public by default
};

The C++ working draft N4950 states: “Members of a class defined with the keyword class are private by default. Members of a class defined with the keywords struct or union are public by default.” WG21 C++ working draft N4950

What an access label changes

An access label applies to the declarations that follow it until another access label appears or the type definition ends. For example, in Example, hidden is private because it appears before public:; visible is public. The label is not limited to the next declaration.

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Elebase USB to USB C Adapter for iPhone 18 Pro Max,USBC Car Charger Adapter
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Access controls where a member name may be used. A private member is still available to the class’s own member functions and friends; it is not accessible as a public member from unrelated code.

Member access and inheritance have separate defaults

The class and struct keywords also affect inheritance when the base-specifier omits an access level. This is separate from the default access of members declared in the type body.

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Declaration Default when access is omitted
Member in a class Private
Member in a struct or union Public
Base of a class, such as class Derived : Base Private inheritance
Base of a struct, such as struct Derived : Base Public inheritance

You can write an explicit access level in the base-specifier to choose a different inheritance access.

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Does this apply to every C++ function?

No. The default-member-access rule concerns functions declared as members inside a class, struct, or union. A function declared at namespace scope is not a member and is not made private by the class default.

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