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V104. Implicit type conversion to memsi…
Analyzer Diagnostics
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Contents

V104. Implicit type conversion to memsize type in an arithmetic expression.

Dec. 15, 2011

The analyzer found a possible error inside an arithmetic expression and this error is related to the implicit type conversion to memsize type. The error of an overflow may be caused by the changing of the permissible interval of the values of the variables included into the expression.

The first example.

The incorrect comparison expression. Let's examine the code:

size_t n;
unsigned i;
// Infinite loop (n > UINT_MAX).
for (i = 0; i != n; ++i) { ... }

In this example the error are shown which are related to the implicit conversion of type 'unsigned' to type 'size_t' while performing the comparison operation.

On the 64-bit platform you may have an opportunity to process a larger data size and the value of the variable 'n' may excess the number 'UINT_MAX' (4 Gb). As a result, the condition "i != n" will be always true and that will cause an eternal cycle.

An example of the corrected code:

size_t n;
size_t i;
for (i = 0; i != n; ++i) { ... }

The second example.

char *begin, *end;
int bufLen, bufCount;
...
ptrdiff_t diff = begin - end + bufLen * bufCount;

The implicit conversion of type 'int' to type 'ptrdiff_t' often indicates an error. One should pay attention that the conversion takes place not while performing operator "=" (for the expression "begin - end + bufLen * bufCount" has type 'ptrdiff_t'), but inside this expression. The subexpression "begin - end" according to C++ rules has type 'ptrdiff_t', and the right "bufLen * bufCount" type 'int'. While changing over to 64-bit platform the program may begin to process a larger data size which may result in an overflow while determining the subexpression "bufLen * bufCount".

You should change the type of the variables 'bufLen' and 'bufCount' into memsize type or use the explicit type conversion, as follows:

char *begin, *end;
int bufLen, bufCount;
...
ptrdiff_t diff = begin - end + 
  ptrdiff_t(bufLen) * ptrdiff_t(bufCount);

Let's notice that the implicit conversion to memsize type inside the expressions is not always incorrect. Let's examine the following situation:

size_t value;
char c1, c2;
size_t result = value + c1 * c2;

The analyzer does not show error message although the conversion of type 'int' to 'size_t' occurs in this case, for there can be no overflow while determining the subexpression "c1 * c2".

If you suspect that the program may contain errors related to the incorrect explicit type conversion in expressions, you may use the V201. Here is an example when the explicit type conversion to type 'size_t' hides an error:

int i;
size_t st;
...
st = size_t(i * i * i) * st;

Additional materials on this topic:

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