What does the explicit keyword mean in C++?
转载于:https://stackoverflow.com/questions/121162/what-does-the-explicit-keyword-mean
The compiler is allowed to make one implicit conversion to resolve the parameters to a function. What this means is that the compiler can use constructors callable with a single parameter to convert from one type to another in order to get the right type for a parameter.
Here's an example class with a constructor that can be used for implicit conversions:
class Foo
{
public:
// single parameter constructor, can be used as an implicit conversion
Foo (int foo) : m_foo (foo)
{
}
int GetFoo () { return m_foo; }
private:
int m_foo;
};
Here's a simple function that takes a Foo object:
void DoBar (Foo foo)
{
int i = foo.GetFoo ();
}
and here's where the DoBar function is called.
int main ()
{
DoBar (42);
}
The argument is not a Foo object, but an int. However, there exists a constructor for Foo that takes an int so this constructor can be used to convert the parameter to the correct type.
The compiler is allowed to do this once for each parameter.
Prefixing the explicit keyword to the constructor prevents the compiler from using that constructor for implicit conversions. Adding it to the above class will create a compiler error at the function call DoBar (42). It is now necessary to call for conversion explicitly with DoBar (Foo (42))
The reason you might want to do this is to avoid accidental construction that can hide bugs. Contrived example:
MyString(int size) class with a constructor that constructs a string of the given size. You have a function print(const MyString&), and you call print(3) (when you actually intended to call print("3")). You expect it to print "3", but it prints an empty string of length 3 instead.Suppose, you have a class String:
class String {
public:
String(int n); // allocate n bytes to the String object
String(const char *p); // initializes object with char *p
};
Now, if you try:
String mystring = 'x';
The character 'x' will be implicitly converted to int and then the String(int) constructor will be called. But, this is not what the user might have intended. So, to prevent such conditions, we shall define the constructor as explicit:
class String {
public:
explicit String (int n); //allocate n bytes
String(const char *p); // initialize sobject with string p
};
This has already been discussed (what is explicit constructor). But I must say, that it lacks the detailed descriptions found here.
Besides, it is always a good coding practice to make your one argument constructors (including those with default values for arg2,arg3,...) as already stated. Like always with C++: if you don't - you'll wish you did...
Another good practice for classes is to make copy construction and assignment private (a.k.a. disable it) unless you really need to implement it. This avoids having eventual copies of pointers when using the methods that C++ will create for you by default. An other way to do this is derive from boost::noncopyable.
The explicit-keyword can be used to enforce a constructor to be called explicitly.
class C{
public:
explicit C(void) = default;
};
int main(void){
C c();
return 0;
}
the explicit-keyword in front of the constructor C(void) tells the compiler that only explicit call to this constructor is allowed.
The explicit-keyword can also be used in user-defined type cast operators:
class C{
public:
explicit inline operator bool(void) const{
return true;
}
};
int main(void){
C c;
bool b = static_cast<bool>(c);
return 0;
}
Here, explicit-keyword enforces only explicit casts to be valid, so bool b = c; would be an invalid cast in this case. In situations like these explicit-keyword can help programmer to avoid implicit, unintended casts. This usage has been standardized in C++11.
This answer is about object creation with/without an explicit constructor since it is not covered in the other answers.
Consider the following class without an explicit constructor:
class Foo
{
public:
Foo(int x) : m_x(x)
{
}
private:
int m_x;
};
Objects of class Foo can be created in 2 ways:
Foo bar1(10);
Foo bar2 = 20;
Depending upon the implementation, the second manner of instantiating class Foo may be confusing, or not what the programmer intended. Prefixing the explicit keyword to the constructor would generate a compiler error at Foo bar2 = 20;.
It is usually good practice to declare single-argument constructors as explicit, unless your implementation specifically prohibits it.
Note also that constructors with
can both be used as single-argument constructors. So you may want to make these also explicit.
An example when you would deliberately not want to make your single-argument constructor explicit is if you're creating a functor (look at the 'add_x' struct declared in this answer). In such a case, creating an object as add_x add30 = 30; would probably make sense.
Here is a good write-up on explicit constructors.
Constructors append implicit conversion. To suppress this implicit conversion it is required to declare a constructor with a parameter explicit.
In C++11 you can also specify an "operator type()" with such keyword http://en.cppreference.com/w/cpp/language/explicit With such specification you can use operator in terms of explicit conversions, and direct initialization of object.
P.S. When using transformations defined BY USER (via constructors and type conversion operator) it is allowed only one level of implicit conversions used. But you can combine this conversions with other language conversions
Cpp Reference is always helpful!!! Details about explicit specifier can be found here. You may need to look at implicit conversions and copy-initialization too.
Quick look
The explicit specifier specifies that a constructor or conversion function (since C++11) doesn't allow implicit conversions or copy-initialization.
Example as follows:
struct A
{
A(int) { } // converting constructor
A(int, int) { } // converting constructor (C++11)
operator bool() const { return true; }
};
struct B
{
explicit B(int) { }
explicit B(int, int) { }
explicit operator bool() const { return true; }
};
int main()
{
A a1 = 1; // OK: copy-initialization selects A::A(int)
A a2(2); // OK: direct-initialization selects A::A(int)
A a3 {4, 5}; // OK: direct-list-initialization selects A::A(int, int)
A a4 = {4, 5}; // OK: copy-list-initialization selects A::A(int, int)
A a5 = (A)1; // OK: explicit cast performs static_cast
if (a1) cout << "true" << endl; // OK: A::operator bool()
bool na1 = a1; // OK: copy-initialization selects A::operator bool()
bool na2 = static_cast<bool>(a1); // OK: static_cast performs direct-initialization
// B b1 = 1; // error: copy-initialization does not consider B::B(int)
B b2(2); // OK: direct-initialization selects B::B(int)
B b3 {4, 5}; // OK: direct-list-initialization selects B::B(int, int)
// B b4 = {4, 5}; // error: copy-list-initialization does not consider B::B(int,int)
B b5 = (B)1; // OK: explicit cast performs static_cast
if (b5) cout << "true" << endl; // OK: B::operator bool()
// bool nb1 = b2; // error: copy-initialization does not consider B::operator bool()
bool nb2 = static_cast<bool>(b2); // OK: static_cast performs direct-initialization
}
Explicit conversion constructors (C++ only)
The explicit function specifier controls unwanted implicit type conversions. It can only be used in declarations of constructors within a class declaration. For example, except for the default constructor, the constructors in the following class are conversion constructors.
class A
{
public:
A();
A(int);
A(const char*, int = 0);
};
The following declarations are legal:
A c = 1;
A d = "Venditti";
The first declaration is equivalent to A c = A( 1 );.
If you declare the constructor of the class as explicit, the previous declarations would be illegal.
For example, if you declare the class as:
class A
{
public:
explicit A();
explicit A(int);
explicit A(const char*, int = 0);
};
You can only assign values that match the values of the class type.
For example, the following statements are legal:
A a1;
A a2 = A(1);
A a3(1);
A a4 = A("Venditti");
A* p = new A(1);
A a5 = (A)1;
A a6 = static_cast<A>(1);