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c++auto

Is it possible to declare auto variables with an if?

My Code is below.

struct conv{
    struct des
    {
        des(int a) {}
        des(int a, int b) {}
        des(int a, int b, int c) {}
    };
};
int main(int argc, const char * argv[]) {

int a = 1;
int b = 1;
int c = 1;

if (a > 1)
{
    auto conv_desc = conv::des(1, 2, 3);
} else if(b > 1) {
    auto conv_desc = conv::des(1, 2);
} else {
    auto conv_desc = conv::des(1);
}
    return 0;
}

The code's pattern is extracted from mkldnn. The only thing I want to do is take auto conv_desc out of the if-else statement. I tried to declare auto conv_desc out of the if-else statement. It occurred an error: Declaration of

variable 'conv_desc' with deduced type 'auto' requires an initializer

Or if I used the pointer like below, I got a null pointer. enter image description here

Another way got an error: enter image description here

Taking the address of a temporary object of type 'conv::des'

If I can't solve this problem, I will have to write a large piece of duplicate code in each branch.

Solution

  • Move your if code into separate function:

    conv::des make_des(int a, int b) {
  if (a > 1) {
    return conv::des(1, 2, 3);
  } else if(b > 1) {
    return conv::des(1, 2);
  } else {
    return conv::des(1);
  }
}

int main(int argc, const char * argv[]) {
  int a = 1;
  int b = 1;
  int c = 1;
  auto conv_desc = make_des(a, b);
  return 0;
}