Something that I have discovered over the years is a great pleasure. When I am giving information – presenting, teaching, writing an article or a blog – it is not necessarily a one-way process. I often receive useful and interesting information back. I have commented that I learn as much from delivering a class as I might from attending one.<\/p>\n
I was recently talking about C++ for embedded at a conference and considering some of the features that I felt resulted in better, clearer and more bug-free code. One of these was the option to pass parameters by reference. Then someone explained a drawback of this feature …<\/p>\n
Pointers are a very powerful feature of the C language and largely explain its wide usage for embedded programming. But they can also be confusing and error prone. For example, consider this code:<\/p>\n
int *p;\np = (int *)0x8000;\np += 1; \/\/ this could equally be p=p+1 or p++<\/pre>\nWhat is the final value of p<\/strong>?<\/p>\n
Assuming the compiler in use regards integers as 32 bits, the answer is 0x8004<\/strong>.<\/p>\n
Although, to the experienced programmer, this may be crystal clear, it is far from obvious to someone coming to the language for the first time.<\/p>\n
Of course, C++ has pointers too and this example behaves in the same way, but C++ has some opportunities to avoid the use of pointers and, hence, write clearer code. And one of those is the option to pass parameters by reference.<\/p>\n
This code might be written in C, for example:<\/p>\n
void swap(int *a, int *b)\n{\n int temp = *a;\n *a = *b;\n *b = temp;\n}<\/pre>\nThe parameters are pointers to data in the calling function and this code is littered with pointer operations, every one of which is a potential error. So, in C++ we can pass the parameters by reference, thus:<\/p>\n
void swap(int& a, int& b)\n{\n int temp = a;\n a = b;\n b = temp;\n}<\/pre>\nNow, the calling function does not need to provide [explicit] pointers and the pointer operations are eliminated in the swap()<\/strong> function, rendering it more readable and less likely to contain a bug.<\/p>\n
This is all fine, but the problem that was pointed out to me was with a call to swap()<\/strong>. In C, the call would look like this:<\/p>\n
swap(&x, &y);<\/pre>\nIt is 100% clear that pointers have been passed and, hence, the possibility that the values of x<\/strong> and y<\/strong> might be changed by the call to swap()<\/strong> is quite clear. However, in C++, if reference parameters were used, the call would be written like this:<\/p>\n
swap(x, y);<\/pre>\nAnd there is no clear indication, without resort to the function prototype [which would, of course, be readily on hand if you are using a good IDE], that x<\/strong> and y<\/strong> could get changed.<\/p>\n
I guess it is a case of “what you win on the swings, you lose on the roundabouts”.<\/p>\n","protected":false},"excerpt":{"rendered":"
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