ifstream::read and pointers to std::vector

Alex Vinokur

Testsuites "Comparativ e Performance Measurement. Reading file into
string" at
http://groups.google.com/group/perfo...73f4d1a05cfbd1
http://groups.google.com/group/sourc...a9b6f91239c909
contain several algorithms including the following ones.

=============== =============== ====
ifstream ifs; // input file stream
string ret_str;

### CPP-21: std::vector and std::copy
------------------------------*------------------
vector<char> v (no_of_file_byt es);
ifs.read(&v[0], no_of_file_byte s);
ostream_iterato r<char> out(oss);
copy (&v[0], &v[v.size()], out);
ret_str = oss.str();
------------------------------*------------------
### CPP-23: std::vector and istream::read()
------------------------------*------------------
vector<char> v (no_of_file_byt es);
ifs.read(&v[0], no_of_file_byte s);
ret_str = (v.empty() ? string() : string (v.begin(), v.end()));
------------------------------*------------------
=============== =============== ====

Eljay Love-Jensen wrote me in private message:

std::vector does not guarantee that the memory is one contiguous block.
Your particular implementation may-or-may-not provide that guarantee -- but
reliance upon that particular implementation behavior is not portable.

So, my algorithms CPP-21 and CPP-23 are not legal because the following
two line are not legal (?).

vector<char> v (no_of_file_byt es);
ifs.read(&v[0], no_of_file_byte s);

So, we can't use read() with pointers to std::vector (?).
--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

Aug 10 '05 #1

Subscribe Reply

3319

Alipha

Alex Vinokur wrote:

Testsuites "Comparativ e Performance Measurement. Reading file into
string" at
http://groups.google.com/group/perfo...73f4d1a05cfbd1
http://groups.google.com/group/sourc...a9b6f91239c909
contain several algorithms including the following ones.

=============== =============== ====
ifstream ifs; // input file stream
string ret_str;

### CPP-21: std::vector and std::copy
------------------------------*------------------
vector<char> v (no_of_file_byt es);
ifs.read(&v[0], no_of_file_byte s);
ostream_iterato r<char> out(oss);
copy (&v[0], &v[v.size()], out);
ret_str = oss.str();
------------------------------*------------------
### CPP-23: std::vector and istream::read()
------------------------------*------------------
vector<char> v (no_of_file_byt es);
ifs.read(&v[0], no_of_file_byte s);
ret_str = (v.empty() ? string() : string (v.begin(), v.end()));
------------------------------*------------------
=============== =============== ====

Eljay Love-Jensen wrote me in private message:
std::vector does not guarantee that the memory is one contiguous block.
Your particular implementation may-or-may-not provide that guarantee --but
reliance upon that particular implementation behavior is not portable.

as of 2003, the C++ standard explicitly states that a std::vector uses
one contiguous block:

ISO/IEC 14882:2003, section 23.2.4-1:

"The elements of a vector are stored contiguously, meaning that if v is
a vector<T, Allocator> where T is some type other than bool, then it
obeys the identity &v[n] == &v[0] + n for all 0 <= n < v.size()."
So, my algorithms CPP-21 and CPP-23 are not legal because the following
two line are not legal (?).

vector<char> v (no_of_file_byt es);
ifs.read(&v[0], no_of_file_byte s);

So, we can't use read() with pointers to std::vector (?).
as of 2003, the above code is legal.

--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

Aug 10 '05 #2

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