The Microsoft Edge team @MSEdgeDev is
hiring C++ Devs! http://bit.ly/1MJxEAX , help us make the web better!
I have no words (C++) http://tgceec.tumblr.com/post/74534916370/results-of-the-grand-c-error-explosion …
StackOverflow Qs on the little known C++ "operators" --> and =! http://stackoverflow.com/questions/1642028 … http://stackoverflow.com/questions/21029174 …
Custom named operators in C++ https://github.com/klmr/named-operator … This is some kind of hack! Depends solely on operator overloading.
Some time back while I was working on getting the Javascript Windows Store app platform running on Windows Phone (now available on the last Windows Phone release!) I had an interesting bug that in retrospect is amusing.
I had just finished a work item to get accessibility working for JS WinPhone apps when I got a new bug: With some set of JS apps, accessibility appeared to be totally broken. At that time in development the only mechanism we had to test accessibility was a test tool that runs on the PC, connects to the phone, and dumps out the accessibility tree of whatever app is running on the phone. In this bug, the tool would spin for a while and then timeout with an error and no accessibility information.
My first thought was this was an issue in my new accessibility code. However, debugging with breakpoints on my code I could see none of my code was run nor the code that should call it. The code that called that code was a more generic messaging system that hit my breakpoints constantly.
Rather than trying to work backward from the failure point, I decided to try and narrow down the repro and work forwards from there. One thing all the apps with the bug had in common was their usage of WinJS, but not all WinJS apps demonstrated the issue. Using a binary search approach on one such app I removed unrelated app code until all that was left was the app's usage of the WinJS AppBar and the bug still occurred. I replaced the WinJS AppBar usage with direct usage of the underlying AppBar WinRT APIs and continued.
Only some calls to the AppBar WinRT object produced the issue:
var appBar = Windows.UI.WebUI.Core.WebUICommandBar.getForCurrentView();
// appBar.opacity = 1;
// appBar.closeDisplayMode = Windows.UI.WebUI.Core.WebUICommandBarClosedDisplayMode.default;
appBar.backgroundColor = Windows.UI.Colors.white; // Bug!
I eventually realized that predefined WinRT color values like Windows.UI.Colors.aqua would cause the issue while JS literal based colors didn't cause the issue (Windows.UI.Color is a WinRT struct which projects in JS as a JS literal object with the struct members as JS
object properties so its easy to write something like {r: 0, g: 0, b: 0, a: 0} to make a color) and I had been mixing both in my tests without realizing there would be a difference.
I debugged into the backgroundColor property setter that consumed the WinRT color struct to see what was different between Windows.UI.Colors.black and {a: 1, r: 0, g: 0, b: 0} and
found the two structs to be byte wise exactly the same.
On a hunch I tried my test app with only a reference to the color and otherwise no interaction with the AppBar and not doing anything with the actual reference to the color:
Windows.UI.Colors.black;. This too caused the issue. I knew that the implementation for these WinRT const values live in a DLL and guessed that something in the code to create these
predefined colors was causing the issue. I debugged in and no luck. Now I also have experienced crusty code that would do exciting things in its DllMain, the function that's called when a DLL is loaded into the process so I tried modifying my
C++ code to simply LoadLibrary the DLL containing the WinRT color definition, windows.ui.xaml.dll and found the bug still occurred! A short lived moment of relief as the world seemed to make
sense again.
Debugging into DllMain nothing interesting happened. There were interesting calls in there to be sure, but all of them behind conditions that were false. I was again stumped. On another hunch I tried renaming the DLL and only LoadLibrary'ing it and the bug went away. I took a different DLL renamed it windows.ui.xaml.dll and tried LoadLibrary'ing that and the bug came back. Just the name of the DLL was causing the issue.
I searched for the DLL name in our source code index and found hits in the accessibility tool. Grinning I opened the source to find that the accessibility tool's phone side service was trying to determine if a process belonged to a XAML app or not because XAML apps had a different accessibility contract. It did this by checking to see if windows.ui.xaml.dll was loaded in the target process.
At this point I got to fix my main issue and open several new bugs for the variety of problems I had just run into. This is a how to on writing software that is difficult to debug.
After much deliberation, the winners of the Grand C++ Error Explosion Competition are finally selected. There are two different award categories. The winners of the first category are those submissions that produced the largest error with the smallest amount of source code. These entries contain a…
|
WinRT (JS and
C++)
|
JS Only
|
C++ Only
|
.NET Only
|
|
|
Parse
|
|
|||
|
Build
|
||||
|
Normalize
|
||||
|
Equality
|
|
|
||
|
Relative
resolution
|
||||
|
Encode data for
including in URI property
|
||||
|
Decode data extracted
from URI property
|
||||
|
Build Query
|
||||
|
Parse Query
|
TL;DR: Keep your C++ class member declaration order the same as your constructor member initializers order.
C++ guarantees that the member initializers in a constructor are called in order. However the order in which they are called is the order in which the associated members are declared in the class, not the order in which they appear in the member initializer list. For instance, take the following code. I would have thought it would print "three, one, two", but in fact it prints, "one, two, three".
#include "stdafx.h"
#include
class PrintSomething {
public:
PrintSomething(const wchar_t *name) { std::wcout << name << std::endl; }
};
class NoteOrder {
public:
// This order doesn't matter.
NoteOrder() : three(L"three"), one(L"one"), two(L"two") { }
PrintSomething one;
PrintSomething two;
PrintSomething three;
};
int wmain(const int argc, const wchar_t* argv[])
{
NoteOrder note; // Prints one, two, three, not three, one, two!
return 0;
}
If you want to represent a value larger than 32bits in an enum in MSVC++ you can use C++0x style syntax to tell the compiler exactly what kind of integral type to store the enum values. Unfortunately by default an enum is always 32bits, and additionally while you can specify constants larger than 32bits for the enum values, they are silently truncated to 32bits.
For instance the following doesn't compile because Lorem::a and Lorem::b have the same value of '1':
enum Lorem {
a = 0x1,
b = 0x100000001
} val;
switch (val) {
case Lorem::a:
break;
case Lorem::b:
break;
}
Unfortunately it is not an error to have b's constant truncated, and the previous without the switch statement does compile just fine:
enum Lorem {
a = 0x1,
b = 0x100000001
} val;
But you can explicitly specify that the enum should be represented by a 64bit value and get expected compiling behavior with the following:
enum Lorem : UINT64 {
a = 0x1,
b = 0x100000001
} val;
switch (val) {
case Lorem::a:
break;
case Lorem::b:
break;
}
Breakdown of the STL’s implementation of next_permutation. Ever wondered how that works?
The following code compiled just fine but did not at all act in the manner I expected:
BOOL CheckForThing(__in CObj *pObj, __in IFigMgr* pFigMgr, __in_opt LPCWSTR url)
{
BOOL fCheck = FALSE;
if (SubCheck(pObj))
{
...bool SubCheck(const CObj& obj);Did you spot the bug? As you can see I should be passing in *pObj not pObj since the method takes a const CObj& not a CObj*. But then why does it compile?
It works because CObj has a constructor with all but one param with default values and CObj is derived from IUnknown:
CObj(__in_opt IUnknown * pUnkOuter, __in_opt LPCWSTR pszUrl = NULL);The lesson is unless you really want this behavior, don't make constructors with all but 1 or 0 default parameters. If you need to do that consider using the 'explicit' keyword on the constructor.
More info about forcing single argument constructors to be explicit is available on stack overflow.
Summary of some of the new C++ features with comments and suggested usage. Not sure I agree with the take on auto.
‘“C++11 feels like a new language.” – Bjarne Stroustrup’
CreateIUriBuilder(resolvedUri, 0, 0, &builder);
builder->SetHost(host);
builder->CreateUri(0xFFFFFFFF, 0, 0, &resolvedUri); ResolveHost(resolvedUri, &resolvedUri); operator T**()
{
T *ptrValue = mPtrValue;
mPtrValue->Release();
mPtrValue = NULL;
return &ptrValue;
}
Some rights reserved