Articles & Books

Quick Q: Why can noexcept generate faster code than throw()?—StackOverflow

Quick A: Because noexcept doesn't have to keep track of stack unwinding.

noexcept, stack unwinding, and performance

The following draft from Scott Meyers new C++11 book says (page 2, lines 7-21)

The difference between unwinding the call stack and possibly unwinding it has a surprisingly large impact on code generation. In a noexcept function, optimizers need not keep the runtime stack in an unwindable state if an exception would propagate out of the function, nor must they ensure that objects in a noexcept function are destroyed in the inverse order of construction should an exception leave the function. The result is more opportunities for optimization, not only within the body of a noexcept function, but also at sites where the function is called. Such flexibility is present only for noexcept functions. Functions with “throw()” exception specifications lack it, as do functions with no exception specification at all.

In contrast, section 5.4 of "Technical Report on C++ Performance" describes the "code" and "table" ways of implementing exception handling. In particular, the "table" method is shown to have no time overhead when no exceptions are thrown and only has a space overhead.

My question is this - what optimizations is Scott Meyers talking about when he talks of unwinding vs possibly unwinding? Why don't these optimizations apply for throw()? Do his comments apply only to the "code" method mentioned in the 2006 TR?

Variadic Templates—Feabhas

variadic-templates.PNGRecently on StickyBits, a nice primer on variadics:

Variadic Templates

by feabhas

From the article:

In this article we’re going to look at a new feature of templates in C++11 -- the concept of the variadic template.

Variadic templates allow us to create functions and classes, not only with generic types, but also a variable number of generic types.

If you haven’t been following along with the template articles, I’d suggest you read this article first before continuing.

A Grand Introduction: Stepanov introduces Stroustrup at CppCon—Tony DaSilva

Bulldozer00's appreciation for Alex Stepanov's introduction for Bjarne Stroustrup at CppCon. We already linked to the video last week -- but if you didn't watch it then, do yourself a favor and spend 6 minutes now getting your workweek off to a great start.

A Grand Introduction

by Tony DaSilva

From the article:

"We should not be ashamed of bits. We should be proud of them." -- Alex Stepanov

You may not interpret it in the same way as I did, but I found this cppcon conference introduction of Bjarne Stroustrup by programming scholar Alex Stepanov very moving...

Quick Q: Can computing the length of a C string really be compile-time constexpr?—StackOverflow

Quick A: Yes, when the string being traversed is itself a constant expression, such as a string literal.

Recently on StackOverflow:

Computing length of a C string at compile time. Is this really a constexpr?

I'm trying to compute the length of a string literal at compile time. To do so I'm using following code:

#include <cstdio>

int constexpr length(const char* str)
{
    return *str ? 1 + length(str + 1) : 0;
}

int main()
{
    printf("%d %d", length("abcd"), length("abcdefgh"));
}

Everything works as expected, the program prints 4 and 8. The assembly code generated by clang shows that the results are computed at compile time:

0x100000f5e:  leaq   0x35(%rip), %rdi          ; "%d %d"
0x100000f65:  movl   $0x4, %esi
0x100000f6a:  movl   $0x8, %edx
0x100000f6f:  xorl   %eax, %eax
0x100000f71:  callq  0x100000f7a               ; symbol stub for: printf

My question: is it guaranteed by the standard that length function will be evaluated compile time?

If this is true the door for compile time string literals computations just opened for me... for example I can compute hashes at compile time and many more...

Quick Q: Why can I return a unique_ptr by value?—StackOverflow

Quick A: Because return local_obj; automatically treats it as an rvalue. After all, you won't be using it any more.

When this FAQ came up again recently on SO, the answer was to refer to this previous Q&A:

Returning unique_ptr from functions

unique_ptr<T> does not allow copy construction, instead it supports move semantics. Yet, I can return a unique_ptr<T> from a function and assign the returned value to a variable...

unique_ptr<int> foo()
{
  unique_ptr<int> p( new int(10) );

  return p;                   // 1
  //return move( p );         // 2
}

The code above compiles and works as intended. So how is it that line 1 doesn't invoke the copy constructor and result in compiler errors? If I had to use line 2 instead it'd make sense (using line 2 works as well, but we're not required to do so)....

Exceptions, error codes, and assertions in C++—Joseph Mansfield

mansfield.pngOne reasoned take on the various error reporting mechanisms in C++ and a policy for deciding when each is appropriate:

Exceptions, error codes, and assertions in C++

by Joseph Mansfield

From the article:

It can often be difficult to decide between the various methods of error reporting in C++. For example, some common advice is that exceptions should only be thrown in exceptional circumstances. Needless to say, this isn't particularly helpful. What exactly is an exceptional circumstance? An exception to what? If we throw assertions into the mix, this can become even more complicated.

In general, functions express a contract to the calling code...

First community planning session was a full success

Yesterday I held the first planning session for local C++ communities at the #meetingcpp chat at freenode.My plan is to make this a monthly online meeting, where new user groups can be planned and existing user groups can exchange and connect.

First community planning session was a full success

by Jens Weller

From the article:

Yesterday I held the first planning session for local C++ communities at the #meetingcpp chat at freenode.My plan is to make this a monthly online meeting, where new user groups can be planned and existing user groups can exchange and connect. As this was the first time, and I just returned from CppCon, so I only could announce it a day before. Yet it was a good start, especially the contact to the brazilian C++ User Group is great to have.

Trip Report: My Trip to CppCon—Jens Weller

From the organizer of Meeting C++:

My Trip to CppCon

by Jens Weller

From the article:

I just returned yesterday from my trip to Bellevue/WA for CppCon. Its been 10 awesome days, and a ton of fun. It was an honor to support this idea from day one as a community sponsor. I already posted some images of CppCon 2014 at facebook and G+.

... And then from Monday till Friday there was the awesome content. I had often a hard time to choose which talk to see next, as with 6 Tracks there often was a scheduling conflict. All the keynotes delivered good content and Mike Acton really gave us something to talk about. One of the highlights in the talks was for me the talk about using boost fusion for wire protocols, ...

Marking as Deprecated in C++14—Joseph Mansfield

Now that C++14 is preparing for publication, get the details on the new [[deprecated]] attribute.

Marking as Deprecated in C++14

by Joseph Mansfield

From the article:

It is common for entities in source code (functions, classes, etc.) to become obsolete or unsafe as a project undergoes development. It's usually a bad idea to remove those entities without any warning, as it'll break any code that interfaces with those entities. Instead, a good practice is to mark them as deprecated in an attempt to discourage their use.

The upcoming C++ release, C++14, introduces the deprecated attribute for specifying that an entity is deprecated.