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Fix move constructor and operator= in String. #6261

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Fix move constructor and operator= in String. #6261

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2da21ad
Fix move constructor and operator= in String.
leg0 May 5, 2017
a30f73b
Remove redundant null checks.
leg0 Jan 19, 2018
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55 changes: 22 additions & 33 deletions hardware/arduino/avr/cores/arduino/WString.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -43,16 +43,15 @@ String::String(const __FlashStringHelper *pstr)
*this = pstr;
}

#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#if STRING_HAS_MOVE
String::String(String &&rval)
: buffer(rval.buffer)
, capacity(rval.capacity)
, len(rval.len)
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Really these should all have a call to std::move, but of course std::move is not available on the arduino.

Perhaps it would be sensible to provide std::move and std::forward if arduino is going to start using rvalue references internally

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Even if std::move was available it would not make much sense to use it here - all it does is to cast is argument to rvalue reference. For integral types and pointers move construction is same as copy construction.

I agree that it would be useful to have move and forward available in lib.

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I guess having std::move on all these assignments would allow move semantics / optimizations on each of the fields, but since we know these are just a pointer and two integers, it should not make any difference here. So I guess the code is good as the PR proposes it.

{
init();
move(rval);
}
String::String(StringSumHelper &&rval)
{
init();
move(rval);
rval.buffer = NULL;
rval.capacity = 0;
rval.len = 0;
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No need for these last two lines - all that matters is that buffer is NULL to prevent rval::~String() from freeing it

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According to c++ standard, object needs to be in valid state after being moved from. Having null buffer and non-zero capacity and /or length does not seem like a valid state to me.

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Keep the state sane is probably a good idea, so I'd suggest leaving these in.

}
#endif

Expand Down Expand Up @@ -137,7 +136,7 @@ inline void String::init(void)

void String::invalidate(void)
{
if (buffer) free(buffer);
free(buffer);
buffer = NULL;
capacity = len = 0;
}
Expand Down Expand Up @@ -189,25 +188,21 @@ String & String::copy(const __FlashStringHelper *pstr, unsigned int length)
return *this;
}

#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#if STRING_HAS_MOVE
void String::move(String &rhs)
{
if (buffer) {
if (rhs && capacity >= rhs.len) {
strcpy(buffer, rhs.buffer);
len = rhs.len;
rhs.len = 0;
return;
} else {
free(buffer);
}
if (this != &rhs)
{
free(buffer);

buffer = rhs.buffer;
len = rhs.len;
capacity = rhs.capacity;

rhs.buffer = NULL;
rhs.len = 0;
rhs.capacity = 0;
}
buffer = rhs.buffer;
capacity = rhs.capacity;
len = rhs.len;
rhs.buffer = NULL;
rhs.capacity = 0;
rhs.len = 0;
}
#endif

Expand All @@ -221,16 +216,10 @@ String & String::operator = (const String &rhs)
return *this;
}

#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#if STRING_HAS_MOVE
String & String::operator = (String &&rval)
{
if (this != &rval) move(rval);
return *this;
}

String & String::operator = (StringSumHelper &&rval)
{
if (this != &rval) move(rval);
move(rval);
return *this;
}
#endif
Expand Down
12 changes: 7 additions & 5 deletions hardware/arduino/avr/cores/arduino/WString.h
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,10 @@ class __FlashStringHelper;
// result objects are assumed to be writable by subsequent concatenations.
class StringSumHelper;

#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#define STRING_HAS_MOVE 1
#endif

// The string class
class String
{
Expand All @@ -59,9 +63,8 @@ class String
String(const char *cstr = "");
String(const String &str);
String(const __FlashStringHelper *str);
#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#if STRING_HAS_MOVE
String(String &&rval);
String(StringSumHelper &&rval);
#endif
explicit String(char c);
explicit String(unsigned char, unsigned char base=10);
Expand All @@ -86,9 +89,8 @@ class String
String & operator = (const String &rhs);
String & operator = (const char *cstr);
String & operator = (const __FlashStringHelper *str);
#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#if STRING_HAS_MOVE
String & operator = (String &&rval);
String & operator = (StringSumHelper &&rval);
#endif

// concatenate (works w/ built-in types)
Expand Down Expand Up @@ -205,7 +207,7 @@ class String
// copy and move
String & copy(const char *cstr, unsigned int length);
String & copy(const __FlashStringHelper *pstr, unsigned int length);
#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
#if STRING_HAS_MOVE
void move(String &rhs);
#endif
};
Expand Down