C++ array assign error: invalid array assignment

Question

I'm not a C++ programmer, so I need some help with arrays. I need to assign an array of chars to some structure, e.g.

struct myStructure {
  char message[4096];
};
string myStr = "hello"; // I need to create {'h', 'e', 'l', 'l', 'o'}
char hello[4096];
hello[4096] = 0;
memcpy(hello, myStr.c_str(), myStr.size());
myStructure mStr;
mStr.message = hello;

I get error: invalid array assignment

Why it doesn't work, if mStr.message and hello have the same data type?

Solution 1

Because you can't assign to arrays -- they're not modifiable l-values. Use strcpy:

#include <string>
struct myStructure
{
    char message[4096];
};
int main()
{
    std::string myStr = "hello"; // I need to create {'h', 'e', 'l', 'l', 'o'}
    myStructure mStr;
    strcpy(mStr.message, myStr.c_str());
    return 0;
}

And you're also writing off the end of your array, as Kedar already pointed out.

Solution 2

Why it doesn't work, if mStr.message and hello have the same data type?

Because the standard says so. Arrays cannot be assigned, only initialized.

Solution 3

You need to use memcpy to copy arrays.

Comments

[fredoverflow]

Actually, the arrays mStr.message and hello in Alex's code are lvalues, because the expressions &mStr.message and &hello are valid. (See section 5.3.1 paragraph 3 in the C++ standard.)

[Stuart Golodetz]

Yup, you're right -- sorry. It seems what I should have said was that myStr.message isn't a modifiable l-value.

[Claudiu]

Interesting. What's the motivation for this? I don't see an issue with assignment to arrays constituting copying the memory over.

[MaEtUgR]

@Claudiu My assumption would be it has to do with the fact that the array variable name itself in C refers to the pointer of the start address of the array and the type of the arrray does in principle not contain information about it's length. So a direct assignment would be a pointer assignment which you rarely want and an automatic array copy is not possible because the length is unkown from the type. With memcpy you have to specify the length.