Explicit in C++

Using Explicit in C++

In C++ it is possible to declare constructors for a class, taking a single parameter, and use those constructors for doing type conversion. For example:

class A {
public:
A(int);
};

void f(A) {}

void g()
{
A a1 = 37;

A a2 = A(47);

A a3(57);

a1 = 67;

f(77);
}

A declaration like:

A a1 = 37;

says to call the A(int) constructor to create an A object from the integer value. Such a constructor is called a "converting constructor".

However, this type of implicit conversion can be confusing, and there is a way of disabling it, using a new keyword "explicit" in the constructor declaration:

class A {
public:
explicit A(int);
};

void f(A) {}

void g()
{
A a1 = 37; // illegal

A a2 = A(47); // OK

A a3(57); // OK

a1 = 67; // illegal

f(77); // illegal
}

Using the explicit keyword, a constructor is declared to be
"nonconverting", and explicit constructor syntax is required:

class A {
public:
explicit A(int);
};

void f(A) {}

void g()
{
A a1 = A(37);

A a2 = A(47);

A a3(57);

a1 = A(67);

f(A(77));
}

Note that an expression such as:

A(47)

is closely related to function-style casts supported by C++. For example:

double d = 12.34;

int i = int(d);



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