NSString *testStr1 = @"a";//常量区
NSString *testStr2 = [NSString stringWithString:@"a"];//常量区
NSString *testStr3 = [NSString stringWithFormat:@"b"];//堆区
NSString *testStr4 = [[NSString alloc] initWithString:@"c"];//常量区
NSString *testStr5 = [[NSString alloc] initWithFormat:@"d"];//堆区
NSString *testStr6 = [[NSString alloc] init];//没有意义