OneDefinitionRule is more strict on global variables (which have static duration). You can’t have 2 global variables sharing the same name. Devil is in the details:

As explained in various posts, you declare the same global variable in a header file that’s included in various compilation units, but you allocate storage in exactly one compilation unit. Under a temporary suspension of disbelief, let’s say there are 2 allocated storage for the same global var, how would you update this variable?

With free function f1(), ODR is more relaxed. http://www.drdobbs.com/cpp/blundering-into-the-one-definition-rule/240166489 explains the Lessor ODR vs Greater ODR. My focus today is the Greater ODR therein. The same function can be included via a header file into multiple compilation units and goes into multiple binaries. Two copies of this function are usually identical definitions. If they actually have different definitions, compiler/linker can’t easily notice and are not required to verify, so no build error (!) but you get could strange run time errors.

Java linker is simpler and never cause any problem so I never look into it.

c++variables: !always objects

Every variable that holds data is an object. Objects are created either with static duration (by defining rather than declaring the variable), with automatic duration (declaration alone) or with dynamic duration via new/malloc().

That’s the short version. Here’s the long version:

– stack variables (including function parameters) – each stack object has a name (multiple possible?) i.e. the host variable, like a door plate on the memory location. When you clone a stack variable you get a cloned object. (Advanced — You could create a reference to the stack object, when you pass the host variable by-reference into a function. You should never return a stack variable by reference)

– heap objects – have no name no “host variable” no door plate. They only have addresses. The address could be saved in a “pointer object”, which is a can of worm. (In many cases, the address is passed around without any pointer object)

– non-local static objects — are more tricky. The variable(i.e. name) is there after you declare it, but it is a door plate without a door. It only becomes a door plate on a storage location when you allocate storage i.e. create the object by “defining” the host variable. There’s a one-definition-rule for static objects, so most of the time you first declare the variable without defining it, then you define it elsewhere. See https://bintanvictor.wordpress.com/2017/05/30/declared-but-undefined-variable-in-c/

C++error: declared but undefined variable

I sometimes declare a static field in a header, but fail to define it (i.e. give it storage). It compiles fine and may even link successfully. When you run the executable, you may hit

error loading library /home/nysemkt_integrated_parser.so: undefined symbol: _ZN14arcabookparser6Parser19m_srcDescriptionTknE

Note this is a shared library.
Note the field name is mangled. You can un-mangle it using c++filt:

c++filt _ZN14arcabookparser6Parser19m_srcDescriptionTknE arcabookparser::Parser::m_srcDescriptionTkn

According to Deepak, the binary files only has mangled names. The linker and all subsequent programs deal exclusively with mangled names.

If you don’t use this field, the undefined variable actually will not bother you! I think the compiler just ignores it.

c++class field defined]header yet !local vars obey ODR

Let’s put function declaration/definition aside — much simpler.

Let’s put aside local static/non-static variables — much simpler. Let’s put aside function parameters. They are like local variables.

The word “static” is heavily overloaded and confusing. I will try to avoid it as far as possible.

The tricky/confusing categories are

  • category: static field. Most complex and better discussed in a dedicated post — See https://bintanvictor.wordpress.com/2017/02/07/c-static-field-init-basic-rules/
  • category: file-scope var — i.e. those non-local vars with “static” modifier
  • category: global var declaration — using “extern”
    • definition of the same var — without “extern” or “static”
  • category: non-static class field, same as the classic C struct field <– the main topic in the post. This one is not about declaration/definition of a variable with storage. Instead, this is defining a type!

I assume you can tell a variable declaration vs a variable definition. Our intuition is usually right.

The Aha — [2] pointed out — A struct field listing is merely describing what constitutes a struct type, without actually declaring the existence of any variables, anything to be constructed in memory, anything addressable. Therefore, this listing is more like a integer variable declaration than a definition!

Main reason to keep struct definition in header — compiler need to calculate size of the struct. Scott Meyers discussed this in-depth along with class fwd declaration and pimpl.

See also

global^file-scope variables]c++

(Seldom quizzed…) This topic is all about lingo..

The word “static” is heavily overloaded and confusing. I will try to avoid it as far as possible.

Any object declared outside a block has “static duration” which means (see MSDN) “allocated at compile time not run time”

“Global” means extern linkage i.e. visible from other files. You globalize a variable by removing “static” modifier if any.

http://stackoverflow.com/questions/14349877/static-global-variables-in-c explains the 2+1 variations of non-local object

  • a single “static” variable in a *.cpp file. The “static” makes this variable visible only to this file.
  • an extern variable. I used this many times. Basically in one *.cpp it’s declared without “static” or extern. In other *.cpp files, it’s declared extern without a initial value.
  • wrong way — if you (use a shared header to) declare the same variable “static” in 2 *.cpp files, then each file gets a distinct file-scope variable of the same name. Nonsense.

“File-scope” means internal linkage i.e. visible only within the file. You make a variable file-scope by adding “static”.


http://en.wikipedia.org/wiki/Global_variable#C_and_C.2B.2B says “Note that not specifying static is the same as specifying extern: the default is external linkage” but I doubt it.

I guess there are 3 forms :

  • static int — file-scope, probably not related to “extern”
  • extern int — global declaration of a var already Defined in a single file somewhere else
  • int (without any modifier) — the single definition of a global var

condition.signalAll Usually requires locking

Across languages,

  • notify() doesn’t strictly need the lock;
  • wait() always requires the lock.
    • c++ wait() takes the lock as argument
    • old jdk uses the same object as lock and conditionVar
    • jdk5 makes the lock beget the conditionVar

—- original jdk
100% — notify() must be called within synchronized block, otherwise exception. See https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#notify()

—- jdk 5 onwards
99% — https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/Condition.html#signal() says that An implementation may (and typically does) require the lock acquired by the notifying thread.

Not 100% strict.

— compare await():
100% — On the same page, the await() javadoc is 100% strict on this requirement!

0% — notify_all() method does NOT require holding the lock.  See http://en.cppreference.com/w/cpp/thread/condition_variable/notify_all

–compare wait()
100% — wait(myLock) requires a lock as argument!

100% — Similar to old JDK, the notifying thread must hold the lock. See https://msdn.microsoft.com/en-us/library/system.threading.monitor.pulseall(v=vs.110).aspx


std::string/vector are on heap; reserve() to avoid re-allocation

These objects use expandable arrays. Has to be allocated on heap.

[[Optimized C++]] P70 confirms that std::string can get re-allocated when it grows beyond current capacity.

http://stackoverflow.com/questions/9521629/stdstringss-capacity-reserve-resize-functions compares std::string.resize() vs reserve().

  • Backgrounder: Capacity — allocated capacity is often uninitialized memory reserved for this string.
  • Backgrounder: size — length of the string that’s fully initialized and accessible. Some of the characters (nulls) could be invisible.
  • string.resize() can increase the string’s size by filling in space (at the end) with dummy characters.
    • After resize(55) the string size is 55. All 55 characters are part of the string proper.
    • changes string’s size() but not capacity
  • string.reserve() doesn’t affect string size. This is a request to increase or shrink the “capacity” of the object. I believe capacity (say 77) is always bigger than the size of 55. The 77 – 55 = 22 extra slots allocated are uninitialized! They only get populated after you push_back or insert to the string.