Re: std::valarray as value in std::map

From:
 James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 05 Nov 2007 17:13:44 -0000
Message-ID:
<1194282824.895834.72870@o38g2000hse.googlegroups.com>
On Nov 5, 2:22 pm, Pete Becker <p...@versatilecoding.com> wrote:

On 2007-11-03 10:20:01 -0400, Chris Forone <4...@gmx.at> said:

Chris Forone schrieb:

Chris Forone schrieb:

Hello group,

g++ (3.4.2, mingw):

float init[] = {1.f, 2.f, 3.f};
std::map<std::string, std::valarray<float> > mp;

mp["name"] = std::valarray(init, 3);
mp["name"].size(); // should be 3, but IS 0!

Do i ignored something? Does map not do a copy of value?

HAND, Chris


compilable:

#include <iostream>
#include <map>
#include <valarray>

int main()
{
    float init[] = {1.f, 2.f, 3.f};
    std::map<std::string, std::valarray<float> > mp;
        mp["name"] = std::valarray<float>(init, 3);
        std::cout << mp["name"].size() << std::endl;
}

g++ (4.1.2, ubuntu) has the same behavior...


SOLVED!

.insert does the trick... operator[] only does std-ctor...


No, it doesn't. It just happens to look like it works. The
problem has nothing to do with valarray: you'll see the same
behavior with any type for the value. The problem is in the
index, not the value. Quoted strings are not guaranteed to be
unique, so mp["name"] may be a different map element than some
other mp["name"].


But his map uses std::string as a key, not char const*, so the
string literal will be converted.

I'm not familiar enough with valarray to really comment, but his
initial use of std::map seems correct: the call to mp["name"]
does insert a valarray constructed withe the default
constructor, before returning a reference to the new object; the
assignment operator then does whatever the assignment operator
for valarray does.

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James Kanze (GABI Software) email:james.kanze@gmail.com
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