How to initialize a (complex?) struct

Question

Let's define this struct:

struct MyStruct {
    int firstInt;
    int secondInt;
    char * firstString;
    char * secondString;
};

I'm trying to initialize a struct like this:

MyStruct s = {4, 5, {'a', 'b', 'c'}, "abc"};

But it's not working. Is there any way to do it? (the firstString is required not to have '\\0' at the end)

Answer 1

Since your requirement is to not have a null terminator at the end, you have to use an array for firstString :

struct MyStruct {
    int firstInt;
    int secondInt;
    char firstString[3];
    char * secondString;
};

Then you can initialize it like this:

MyStruct s = {4, 5, {'a', 'b', 'c'}, "abc"};

You cannot initialize a char* with {'a', 'b', 'c'} because you have to provide storage for the characters, a char* is only able to point at something. "abc" happens to be a constant string literal which is stored in read only memory, so you are able to make the char* point at that.

Also, in C++, "abc" is a constant which cannot me modified, so you should change char * secondString; to const char * secondString; .

Answer 2

You can use a compound literal for this:

struct MyStruct {
    int firstInt;
    int secondInt;
    char * firstString;
    char * secondString;
};

struct MyStruct s = { 4, 5, (char[]){'a', 'b', 'c'}, "abc" };

This construct was introduced in C99; see section 6.5.2.5 of the N1256 draft . Some compilers (particularly Microsoft's) may not support it.

Note that in C, the type struct MyStruct cannot be referred to as just MyStruct ; this is a difference between C and C++. Make sure you're compiling the language you think you are.

One thing to watch out for is the lifetime of the object associated with a compound literal. String literals denote array objects with static lifetime, ie, the object exists for the entire execution of the program. The array object associated with (char[]){'a', 'b', 'c'} has static storage duration if it occurs outside the body of a function, but automatic storage duration (associated with the innermost enclosing block) if it occurs in side the body of the function. This could be a problem if you tried to pass a copy of s outside the block in which it's defined.

Answer 3

A subtle point arises. Your struct has two char * but has no storage to back these pointers up. You probably want something like this:

struct MyStruct {
    int firstInt;
    int secondInt;
    char firstString[3];
    char secondString[4];
};

Answer 4

#include <iostream>

using namespace std;

struct MyStruct {
    int firstInt;
    int secondInt;
    char * firstString;
    char * secondString;
};

int main()
{
    char arr[] = {'a', 'b', 'c','\0'};
    MyStruct s = {4, 5, arr, "abc"};

    cout << s.firstString << endl;

    return 0;
}

MyStruct s = {4, 5, {'a', 'b', 'c'}, "abc"};

{'a', 'b', 'c'} have no memory to store.It's a r-value. 

——I think. ^-^