C++11 explicit specifier
What's new in C++
1、C++11 explicit specifier的引入主要是为了解决"safe bool problem"的，这在 operator-bool-and-safe-bool-problem 章节中进行了讨论
2、C++11 explicit specifier的引入给予了programmer对implicit、explicit type conversion的控制，它体现了C++的灵活性。
stackoverflow 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:
#include <iostream>
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;
};
void DoBar(Foo foo)
{
int i = foo.GetFoo();
std::cout << i << std::endl;
}
int main()
{
DoBar(42);
}
// g++ --std=c++11 test.cpp
NOTE:
1、输出如下：
422、上述是典型的 implicit conversion + copy initialization + converting constructor
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:
1、You have a 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.
#include <iostream>
class Foo
{
public:
// single parameter constructor, can be used as an implicit conversion
explicit Foo(int foo)
: m_foo(foo)
{
}
int GetFoo()
{
return m_foo;
}
private:
int m_foo;
};
void DoBar(Foo foo)
{
int i = foo.GetFoo();
std::cout << i << std::endl;
}
int main()
{
DoBar( { 42 });
}
// g++ --std=c++11 test.cpp
NOTE: 编译报错如下：
test.cpp: 在函数‘int main()’中: test.cpp:28:15: 错误：从初始化列表转换为‘Foo’将使用显式构造函数‘Foo::Foo(int)’ DoBar( { 42 });
#include <iostream>
class Foo
{
public:
// single parameter constructor, can be used as an implicit conversion
explicit Foo(int foo)
: m_foo(foo)
{
}
int GetFoo()
{
return m_foo;
}
private:
int m_foo;
};
void DoBar(Foo foo)
{
int i = foo.GetFoo();
std::cout << i << std::endl;
}
int main()
{
DoBar(Foo { 42 });
}
NOTE:
1、这个回答的例子非常好
Suppose, you have a class String:
#include <iostream>
class String
{
public:
String(int n) // allocate n bytes to the String object
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
String(const char *p) // initializes object with char *p
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
};
int main()
{
String mystring = 'x';
}
// g++ test.cpp -pedantic -Wall -Wextra
NOTE:
1、输出如下:
String::String(int)2、显然，上述输出是出人意外的
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:
#include <iostream>
class String
{
public:
explicit String(int n) // allocate n bytes to the String object
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
String(const char *p) // initializes object with char *p
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
};
int main()
{
String mystring = 'x';
}
// g++ test.cpp --std=c++11 -pedantic -Wall -Wextra
NOTE:
1、编译报错如下:
test.cpp: In constructor 'String::String(int)': test.cpp:5:22: warning: unused parameter 'n' [-Wunused-parameter] explicit String(int n) // allocate n bytes to the String object ~~~~^ test.cpp: In constructor 'String::String(const char*)': test.cpp:10:21: warning: unused parameter 'p' [-Wunused-parameter] String(const char *p) // initializes object with char *p ~~~~~~~~~~~~^ test.cpp: In function 'int main()': test.cpp:18:20: error: invalid conversion from 'char' to 'const char*' [-fpermissive] String mystring = 'x'; ^~~ test.cpp:10:21: note: initializing argument 1 of 'String::String(const char*)' String(const char *p) // initializes object with char *p2、从上述编译报错中可以看到，此时compiler没有选择
String(int n)
使用场景
1、 explicit主要由于对user-defined conversion的implicit、explicit 进行控制，它主要用于:
a、explicit constructor
b、user-defined conversion function
2、C++17 deduction guide
cppreference explicit specifier
explicit |
1) | |
explicit ( expression ) (since C++20) |
2) |
1) Specifies that
1、constructor
2、conversion function (since C++11)
3、deduction guide (since C++17)
that is, it cannot be used for implicit conversions and copy-initialization.
2)
NOTE: TODO
Example
#include <iostream>
struct A
{
A(int)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
} // converting constructor
A(int, int)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
} // converting constructor (C++11)
operator bool() const
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
return true;
}
};
struct B
{
explicit B(int)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
explicit B(int, int)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
explicit operator bool() const
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
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)
; // 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 (b2)
; // 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
}
// g++ test.cpp -pedantic -Wall -Wextra --std=c++11
Example
libstdc++ unique_ptr
/** Takes ownership of a pointer.
*
* @param __p A pointer to an object of @c element_type
*
* The deleter will be value-initialized.
*/
template <typename _Up = _Dp,
typename = _DeleterConstraint<_Up>>
explicit
unique_ptr(pointer __p) noexcept
: _M_t(__p)
{ }
下面是错误用法
static std::unique_ptr<Api> New(const char *WorkPath)
{
return { NewApi(WorkPath)};
}
In file included from main.cpp:8:
./../common.h:99:10: fatal error: chosen constructor is explicit in copy-initialization
return { NewStockOptApi(WorkPath)};
^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/unique_ptr.h:141:7: note: constructor declared
here
unique_ptr(pointer __p) noexcept