This question already has an answer here:
What is the best practice for casting between the different number types? Types float
, double
, int
are the ones I use the most in C++.
An example of the options where f
is a float
and n
is a double
or an int
:
float f = static_cast<float>(n);
float f = float(n);
float f = (float)n;
I usually write static_cast<T>(...)
but wondered if there was any consensus within the C++ development community if there is a preferred way.
I appreciate this is may end up being an opinion based question and there may not be a "standard" way, in which case please let me know that there is no standard way so at least I know that :-)
I know this question has cropped up in relation to casting in general, however, I am interested specifically in numbers and whether there are specific best practices in the approach for number types.
Just use static_cast
. The problem with C casts is the ambiguity of the operation (i.e. point (1) of Explicit type conversion).
C++ casts avoid this. Additionally C++ casts are more visible when searching for them.
Using Stroustrup's words (What good is static_cast?):
Even an innocent-looking cast can become a serious problem if, during development or maintenance, one of the types involved is changed. For example, what does this mean?:
x = (T)y;
We don't know. It depends on the type
T
and the types of x and y.T
could be the name of a class, atypedef
, or maybe a template parameter. Maybex
andy
are scalar variables and(T)
represents a value conversion. Maybex
is of a class derived fromy
's class and(T)
is a downcast. Maybex
andy
are unrelated pointer types. Because the C-style cast(T)
can be used to express many logically different operations, the compiler has only the barest chance to catch misuses. For the same reason, a programmer may not know exactly what a cast does. This is sometimes considered an advantage by novice programmers and is a source of subtle errors when the novice guessed wrong.The "new-style casts" were introduced to give programmers a chance to state their intentions more clearly and for the compiler to catch more errors.
[CUT]
A secondary reason for introducing the new-style cast was that C-style casts are very hard to spot in a program. For example, you can't conveniently search for casts using an ordinary editor or word processor.
[CUT]
casts really are mostly avoidable in modern C++
Also consider boost::numeric::converter
/ boost::numeric_cast
that are safer alternatives (part of Boost.NumericConversion library).
E.g.
#include <iostream>
#include <boost/numeric/conversion/cast.hpp>
int main()
{
using boost::numeric_cast;
using boost::numeric::bad_numeric_cast;
using boost::numeric::positive_overflow;
using boost::numeric::negative_overflow;
try
{
int i = 42;
short s = numeric_cast<short>(i); // This conversion succeeds (is in range)
}
catch(negative_overflow &e) { std::cout << e.what(); }
catch(positive_overflow &e) { std::cout << e.what(); }
return 0;
}
In general for both implicit conversions and explicit conversions (through static_cast
) the lack of preservation of range makes conversions between numeric types error prone.
numeric_cast
detects loss of range when a numeric type is converted and throws an exception if the range cannot be preserved.
Generally, these casting operators are classified under two major groups: specific casting operators and traditional casting operators. cplusplus.com explains it like this:
...In order to control these types of conversions between classes, we have four specific casting operators: dynamic_cast, reinterpret_cast, static_cast and const_cast. Their format is to follow the new type enclosed between angle-brackets (<>) and immediately after, the expression to be converted between parentheses.
dynamic_cast <new_type> (expression)
reinterpret_cast <new_type> (expression)
static_cast <new_type> (expression)
const_cast <new_type> (expression)
The traditional type-casting equivalents to these expressions would be:
(new_type) expression
new_type (expression)
but each one with its own special characteristics.
While working on a task, we (nearly) all use specific casting. After considering the advises, it's up to you somehow.
See the resource.
来源:https://stackoverflow.com/questions/37497016/best-practice-in-c-for-casting-between-number-types