C++ Error: Incompatible types in assignment of ‘ch

问题:

I have a bit of a problem with my constructor. In my header file I declare:

char short_name_[2]; 

• and other variables

In my constructor:

Territory(std::string name, char short_name[2], Player* owner, char units);
void setShortName(char* short_name);
inline const char (&getShortName() const)[2] { return short_name_; }

In my cpp file:

Territory::Territory(std::string name, char short_name[2], Player* owner, 
                     char units) : name_(name), short_name_(short_name), 
                    owner_(owner), units_(units)
{ }

My error:

Territory.cpp: In constructor ‘Territory::Territory(std::string, char*, Player*, char)’: Territory.cpp:15:33: error: incompatible types in assignment of ‘char*’ to ‘char [2]’

I already figured out that char[2] <=> char* but I'm not sure how to handle this about my constructor and get/setters.

回答1:

Raw arrays in C++ are kind of annoying and fraught with peril. This is why unless you have a very good reason to you should use std::vector or std::array.

First off, as others have said, char[2] is not the same as char*, or at least not usually. char[2] is a size 2 array of char and char* is a pointer to a char. They often get confused because arrays will decay to a pointer to the first element whenever they need to. So this works:

char foo[2];
char* bar = foo;

But the reverse does not:

const char* bar = "hello";
const char foo[6] = bar; // ERROR

Adding to the confusion, when declaring function parameters, char[] is equivalent to char*. So in your constructor the parameter char short_name[2] is really char* short_name.

Another quirk of arrays is that they cannot be copied like other types (this is one explanation for why arrays in function parameters are treated as pointers). So for example I can not do something like this:

char foo[2] = {'a', 'b'};
char bar[2] = foo;

Instead I have to iterate over the elements of foo and copy them into bar, or use some function which does that for me such as std::copy:

char foo[2] = {'a', 'b'};
char bar[2];
// std::begin and std::end are only available in C++11
std::copy(std::begin(foo), std::end(foo), std::begin(bar));

So in your constructor you have to manually copy the elements of short_name into short_name_:

Territory::Territory(std::string name, char* short_name, Player* owner, 
                     char units) : name_(name), owner_(owner), units_(units)
{ 
    // Note that std::begin and std::end can *not* be used on pointers.
    std::copy(short_name, short_name + 2, std::begin(short_name));
}

As you can see this is all very annoying, so unless you have a very good reason you just should use std::vector instead of raw arrays (or in this case probably std::string).

回答2:

When a function wants an array as argument, it gets a pointer to the first element of an array instead. This pointer cannot be used to initialize an array, because it's a pointer, not an array.

You can write functions that accept references to arrays as arguments:

void i_dont_accept_pointers(const char (array&)[2]) {}

The problem here is, that this array reference cannot be used to initialize another array.

class Foo {
  char vars[2];
  Foo(const char (args&)[2])
    : vars(args)  // This will not work
  {}
};

C++ 11 introduced std::array to eliminiate this and other problems of arrays. In older versions, you will have to iterate through the array elements and copy them individually or use std::copy.