On Wednesday, April 19, 2017 at 9:10:16 AM UTC+2, hui zhang wrote:
>
> for 1) you mean
>>
>> char *CGetPath() {
>> return getpath().c_str();
>> }
>
>
> this code will work ?
>
That depends on whether getpath() returns a std::string or a (const)
std::string& (a (const) reference). It will work if it returns a reference
or const reference, but is not guaranteed to work if it doesn't (though it
may appear to do so in some cases, or for some implementations of the C++
standard lib) because the standard lib is not required to use a
reference-counted dynamic array. For example, it might instead choose to
copy the array every time or to allocate small strings using an in-object
buffer to avoid the overhead of dynamic allocation, and in both of those
cases the return value of c_str() is no longer valid when the function
returns.
Also, if do you need to copy the string in C++ code:
strdup(getpath().c_str()) is better than malloc+strcpy (shorter and harder
to get wrong). Case in point: your implementation malloc'ed str.length()
bytes, failing to allocate an extra byte for the '\0' at the end.
Even better might be to call GoString(getpath().c_str()) in the CGetPath
function to ensure only a single copy is made, but I'm not familiar enough
with cgo to know if that's possible.
> 2017-04-19 14:43 GMT+08:00 Konstantin Khomoutov <
> flat...@users.sourceforge.net <javascript:>>:
>
>> On Wed, 19 Apr 2017 14:23:09 +0800
>> hui zhang <fastf...@gmail.com <javascript:>> wrote:
>>
>> > 1) getpath() return a temp string, its c_str() pointer will be
>> > free out of function. So I use malloc
>>
>> I'm not completely sure you're correct. C++ does not implement garbage
>> collection and the object on which you have called c_str() continues to
>> live on after getpath() returns. Now let's cite the manual on c_str():
>>
>> | The pointer obtained from c_str() may be invalidated by:
>> | * Passing a non-const reference to the string to any standard library
>> | function, or
>> | * Calling non-const member functions on the string, excluding
>> | operator[], at(), front(), back(), begin(), rbegin(), end() and
>> | rend().
>>
>> So I'd say in the sequence of calls I propose there's no chance of
>> anything quoted to happen -- basically the pointer returned by c_str()
>> gets passed to Go where C.GoString() copies the memory pointed to by it.
>>
>> Things may break if you somehow concurrently call into your C++ side
>> and those calls may access the same std::string object we're talking
>> about. But if you do this, all bets are already off.
>>
>> > 2) Assume we use the malloc way , are C.GoString() copying the
>> > pointer memory ? for we need C.free() malloc memory.
>>
>> Yes, C.GoString() copies the memory.
>>
>> Yes, you always need to free() what was malloc()ed for you -- because
>> you own the returned pointer.
>>
>
>
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