Casting needed when comparing using brace initialization

[Paul]

In the code below, why isn't {3,4} recognized immediately as a vector<int>?
Why does it fail to compile if I replace if(vec == std::vector<int>({3,4})) by if(vec == {3,4}) ?

Many thanks for your help.

Paul


void printVectorIfSpecial(const std::vector<int>& vec)
{
if(vec == std::vector<int>({3,4}))
{
// do something
}

}

[bartekltg]

There is no type (of the class) deduction based on a constructor.
And this is the main reason, why we have a _function_ make_pair.
pair {1,2} do not work.
Either pair<int> {1,2} or make_pair(1,2)

bartekltg

[bartekltg]

OK, this is wrong answer. A correct one, but for different
question ;-)

{1,2,3} is an iniclializer list, not a vector.
vector i c++ is just z class, not a spacjal type in language.
You can not use vector in you program (but why you would;))
and {1,2} still is the same.

bartekltg

[Martin Shobe]

On 9/30/2015 3:37 AM, Paul wrote:
> In the code below, why isn't {3,4} recognized immediately as a vector<int>?

Because it's not a vector, it's an initializer list.

> Why does it fail to compile if I replace if(vec == std::vector<int>({3,4})) by if(vec == {3,4}) ?

Because there's no equality operator defined for vectors and initializer
lists.

> void printVectorIfSpecial(const std::vector<int>& vec)
> {
> if(vec == std::vector<int>({3,4}))
> {
> // do something
> }
>
> }

Martin Shobe

[Ben Bacarisse]

Paul <peps...@gmail.com> writes:

> In the code below, why isn't {3,4} recognized immediately as a
> vector<int>?

The main problem is this one:

> Why does it fail to compile if I replace if(vec ==
> std::vector<int>({3,4})) by if(vec == {3,4}) ?

It fails to compiler because it's not syntactically valid -- the {...}
form is an initialiser list and that just can't occur in that position.
An IL is not a valid primary expression.

So your questions are really why does C++ not permit an IL to be a
primary expression, and, if C++ did permit it, would it be able to find
the correct operator== function to call?

I can't give you a good answer to either question. I suspect that
making ILs into primary expressions would introduce a vast rats nest of
complications, but that's not much more than a guess.

<snip>
--
Ben.

[Paul]

Thanks, Ben and everyone else. My posting was in no sense intended as a "Why doesn't C++ allow...?" type of question. I didn't know why I got the error and was trying to avoid similar problems in future. So, suppose that I want code which says "do something if the vec is {3, 4}" Is my code (which works) best practice, or should I avoid casting? If so, how?

Many thanks for your help,

Paul

[Alf P. Steinbach]

On 9/30/2015 10:37 AM, Paul wrote:
> In the code below, why isn't {3,4} recognized immediately as a vector<int>?
> Why does it fail to compile if I replace
>
> if(vec == std::vector<int>({3,4}))
>
> by
>
> if(vec == {3,4}) ?
>

> void printVectorIfSpecial(const std::vector<int>& vec)
> {
> if(vec == std::vector<int>({3,4}))
> {
> // do something
> }
>
> }

I'm not sure. Possibly due to pure grammar issues. Consider that the
following works as intended:

<code>
#include <assert.h>
#include <vector>
#include <stdio.h>

void printVectorIfSpecial(const std::vector<int>& vec)
{

if( operator==( vec, {3,4} ) )
{
assert( vec.size() == 2 ); // Doesn't trigger: size is 2.
printf( "{3,4}.size() = %d.\n", int( vec.size() ) );
}
}

auto main() -> int
{
printVectorIfSpecial( std::vector<int>{3,4} );
}
</code>

The assert is just about the possibility that without knowing exactly
what it does, {3,4} /could/ be invoking the 2-argument vector
constructor that would create a vector of 4 items, each of value 3.

But given that the code works, it's hard (for me) to say why your
original isn't permitted.


Cheers, & sorry if that doesn't really help, but,

- Alf

[Ben Bacarisse]

With Alf's answer you have the full picture: the direct comparison is
not permitted for grammatical reasons, but you can do what you want (in
this case at least) by writing the operator== function call directly.

A function call's arguments (in C++11) may include a number of brace
enclosed initializer clauses, but these are not permitted on either side
of an equality operator.

--
Ben.

[Gareth Owen]

Paul <peps...@gmail.com> writes:

> Thanks, Ben and everyone else. My posting was in no sense intended as
> a "Why doesn't C++ allow...?" type of question. I didn't know why I
> got the error and was trying to avoid similar problems in future. So,
> suppose that I want code which says "do something if the vec is {3,
> 4}" Is my code (which works) best practice, or should I avoid casting?

I think partly your difficulty lies in thinking of

std::vector<int>({3,4})

as a cast. It really isn't, it's actually constructing a whole new
object and initialising it with the values.

By analogy, your code is more like:

if(v == std::vector<int>(19,7)) {
// do something if v is a 19 element vector of '7's.
}

So, you might use a const local at function or file scope

namespace{
const std::vector meaningful_name({3,4});
}

int myfunc(void)
{
// or maybe
static const std::vector alternative_meaningful_name({3,4});

if(v == meaningful_name){
// ...
}
}