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Quick Q: Why do I have to write 'mutable' on a lambda? -- StackOverflow

By Blog Staff | Aug 5, 2013 11:04 AM | Tags: intermediate

Inquiring minds want to know:

Why does C++0x's lambda require “mutable” keyword for capture-by-value, by default?

Short example:

#include <iostream>

int main()
{
    int n;
    [&](){n = 10;}();             // OK
    [=]() mutable {n = 20;}();    // OK
    // [=](){n = 10;}();          // Error: a by-value capture cannot be modified in a non-mutable lambda
    std::cout << n << "\n";       // "10"
}

The question: Why do we need the mutable keyword? It's quite different from traditional parameter passing to named functions. What's the rationale behind?

I was under the impression that the whole point of capture-by-value is to allow the user to change the temporary -- otherwise I'm almost always better off using capture-by-reference, aren't I?

Any enlightenments?

(I'm using MSVC2010 by the way. AFAIK this should be standard)

Quick Q: Where do I have to write 'template' and 'typename'? -- StackOverflow

By Blog Staff | Aug 4, 2013 09:19 PM | Tags: intermediate

This question is an oldie but goodie. Note that in C++11 some of this becomes clearer, because it's now recommended to use constexpr and using template aliases instead of "traits" styles, and traits are one of the bigger (but not only) reasons to have a template refer to another template.

Where and why do I have to put the “template” and “typename” keywords?

In templates, where and why do I have to put typename and template on dependent names? What exactly are dependent names anyway? I have the following code: ...

Why Would You Ever Pass a Container By Value? -- Andrew Koenig

By Blog Staff | Jul 31, 2013 03:54 PM | Tags: basics

From the desk of ARK:

Why Would You Ever Pass a Container By Value?

by Andrew Koenig

From the article:

Consider two fundamental features of C++: functions and references. Which shall we teach first?

If we teach references first, there is the problem of coming up with interesting example programs that use references but completely avoid user-defined functions. This is hard to do because the most common use of references is as function parameters — so it's probably easier to teach functions first.

However, if we teach functions before we teach references, then every function we write must accept its arguments by value — references not yet being available as an alternative...

Contextually Converted to Bool -- Chris Sharpe

By Blog Staff | Jul 31, 2013 09:03 AM | Tags: intermediate

A nice writeup of when this "it just works the way you expect" feature kicks in:

Contextually converted to bool

by Chris Sharpe

From the article:

Something I found mildly surprising about C++11 is that this works:

#include <iostream>

struct Testable
{
    explicit operator bool() const { return true; }
};

int main()
{
    Testable t;
    if (t)
        std::cout << "Converted to true!\n";
}

That is, it compiles and prints Converted to true!.

The new bit here is the explicit keyword. When I first saw an example like this, I expected to have to write

if (bool( t )) // ...

[...]

Quick Q: Why do I have to std::move a variable that is already a &&? -- StackOverflow

By Blog Staff | Jul 30, 2013 02:09 PM | Tags: basics

Today on SO, a frequently asked question and a nicely summed up answer:

std::move on a variable which already is T&&

On the page http://msdn.microsoft.com/en-us/library/dd293665.aspx Microsoft has an example on how to write a move constructor. It is essentially of the form:

MyClass::MyClass(MyClass&& lhs)
{
    *this = std::move(lhs);
}

I have tried and std::move really is required here, but why? I thought the only thing move did was to convert to T&&. But lhs is already of type MyClass&&, is it not?

Some Notes on C++11 Lambda Functions -- Ranju V.

By Blog Staff | Jul 30, 2013 09:32 AM | Tags: basics

ranjuv.jpgHere's a nice lambda synopsis with some usage notes.

Some notes on C++11 lambda functions

by Ranju V.

From the article:

Bjarne Stroustrup says that C++11, which is the latest ratified revision of the C++ standard, “feels like a new language”.  I think lambda functions are a big part of what makes the language feel so very different from C++03.  Lambda functions basically allow you to do things like this:

vector<int> nums { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
auto evens = count_if(begin(nums), end(nums), [](int num) {
    return (num % 2) == 0;
});

Is Moving Objects Worth the Hassle? -- Andrew Koenig

By Blog Staff | Jul 26, 2013 11:06 AM | Tags: basics

Koenig's latest just went live at DDJ:

Is Moving Objects Worth the Hassle?

by Andrew Koenig

From the article:

Last week, I discussed how C++ compilers use overloading to decide whether to move or copy an object. This week, I'd like to take a step back and discuss why moving instead of copying is worth doing in the first place.

You might think that this claim needs no justification. After all, copying takes time; so programs that copy a lot of data will be slower than programs that avoid doing so. However, this kind of unthinking optimization can lead to trouble. For example...