C++二维数组定义出现问题.

char choice;
int x,y;
cin>>choice;
if(choice=='1'){
x=5;
y=8;}
else if(choice=='2'){
x=10;
y=10;}
else if(choice=='3'){
x=25;
y=25;
}
char dt[x][y]
你们懂我意思吧
但是出来俩错误
cannot allocate an array of constant size 0.
expected constant expression.**