How to initialize 2-dim array which is a struct member in C++

Question

struct abc {
    double matrix[2][2];
};

int main(){
   abc test;
   test.matrix[2][2]={0,0,0,0};
}

I construct a struct called abc, and 2*2 matrix is its memember. But how to initialize the matrix in main function? The above code always comes with error... how to fix it?

You are technically not initializing in main, you are assigning. See these two: http://stackoverflow.com/questions/4981197/2d-array-values-c http://stackoverflow.com/questions/11733919/how-to-assign-multiple-values-at-once-to-multi-dimensional-array-after-creating — Adam Burry, Sep 09 '13 at 04:21
You should include the compiler version and error message in your question. — Adam Burry, Sep 09 '13 at 04:30
Do you want to initialize it to be full of `0`s, or to some arbitrary set of values? — juanchopanza, Sep 09 '13 at 06:03
@SantoshSahu only works with c99- read: [Strange initializer expression?](http://stackoverflow.com/questions/18329206/strange-initializer-expression/18329258#18329258) — Grijesh Chauhan, Sep 09 '13 at 11:08

Adam Burry · Answer 1 · 2013-09-29T15:44:19.800

0

gcc 4.5.3: g++ -std=c++0x -Wall -Wextra struct-init.cpp

struct abc {
  double matrix[2][2];
};

int main(){
 abc test;
 test.matrix = {{0.0,0.0},{0.0,0.0}};
}

Or maybe simple is best:

struct abc {
  double matrix[2][2];
  abc() {
    matrix[0][0] = 0.0; matrix[0][1] = 0.0;
    matrix[1][0] = 0.0; matrix[1][1] = 0.0; }
};

int main(){
 abc test;
}

edited Sep 29 '13 at 15:44

answered Sep 09 '13 at 04:00

Adam Burry

1,904
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ur code does not work for me... same error occurs for the last line...i use vc++ – user2756494 Sep 09 '13 at 04:06
You have posted neither your exact error message nor your exact compiler version, how much help are you reasonably expecting? – Adam Burry Sep 29 '13 at 13:09

score 0 · Answer 2 · edited May 23 '17 at 12:28

See also this and this

You can write:

struct abc 
{
    int foo;
    double matrix[2][2];
};

void f()
{
   abc test = 
       { 
        0,        // int foo; 
        {0,0,0,0} // double matrix[2][2];
       };

}

I've added foo for clarity on why the additional set of {} around the array.

Note that this kind of structure initialisation can only be used with an aggregate data type, which roughly means a C-link struct.

If you really need to construct and then assign, you may need to do something like:

struct Matrix
{
    double matrix[2][2];
};

struct abc2 
{       
    int foo;
    Matrix m;
};

void g()
{
   abc2 test;
   Matrix init =  { 5,6,7,8};
   test.m = init;
}

How to initialize 2-dim array which is a struct member in C++

2 Answers2