带有const参数的C ++构造函数

Question

I just wondered what's the difference between having a constructor with const char* c or char* const c as a parameter? If I swap char* and const in header and source file it isn't a problem but when I mix them it won't compile, saying there is no such overloaded constructor.

When I use my own class for a copy constructor the compiler doesn't seem to bother if I have MyClass const &other and const MyClass &other mixed..

Can anyone enlighten me please? :)

Answer 1

This record const char *c declares pointer c that points to an object of type const char . That is using this pointer you may not change the object that refered to by the pointer.

This record char * const c declares const pointer c that points to a non-const object of type char . You may not change the pointer itself but you may change the object refered to by the pointer.

There is a difference between declaring pointers and references. Though there is used term "constant reference" strictly speaking references themselves may not be constant. According to the C++ grammar

ptr-operator:
* attribute-specifier-seqopt cv-qualifier-seq
& attribute-specifier-seqopt

references do not contain cv-qualifier-seq. So for example this record

Airplane & const other

will be incorrect and the compiler shall issue an error. As for these records

const Airplane & other
Airplane const & other

then they are equivalent the same way as equivalent the recirds below

const char *c
char const *c

Answer 2

Think of the * as a "fence". What's before it in the declaration defines the type that the pointer refers to . What's after the * defines the pointer itself. For this case, it's easiest to read things backwards (translating * as "pointer to"), so char const *t is read (from back to front) as: "t is a pointer to a const char". Likewise, char *const t is read as: "t is a const pointer to a char."

1. char *const means the pointer itself is const (can't be modified), but the char(s) it points at can be modified.
2. char const * or const char * are equivalent to each other. Both mean the pointer itself can be modified, but what it points at cannot.

Substituting a different type for char doesn't change that basic idea.

The same is true with a reference: const T & means a reference to a const T , so you can't modify the T that it refers to. In the case of a reference, you can't modify the reference itself to refer to a different object, so T & const doesn't make any real sense (and isn't allowed).

Answer 3

const适用于其左侧的类型（如果左侧没有任何内容，则适用于其右侧），因此const char *和char const *相同并且意味着字符不能更改，但指针可以，而char * const表示字符可以更改，但指针不能

Answer 4

The order in which const and char* are written actually make a difference. If you have const char* in your header but char* const in your source file, you will get an error because the compiler won't regard these as the same.

const char* p means that p points to a char that is constant. We can alter where p points to, but we can never alter the contents stored at p.

char* const p means that p is a constant pointer and is pointing to a char. We can alter the contents stored at p, but we cannot alter where p points to.

const char* const p means that p is a constant pointer to a constant char. We cannot alter where p points to nor can we alter the contents at p. Hope this is helpful!