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• Calling constructors in c++ without new

  - by Nils

  I've often seen that people create objects in C++ using Thing myThing("asdf"); Instead of Thing myThing = myThing("asdf"); This seems to work (using gcc), at least as long as there are no templates involved. My question now, is the first line correct and if so should I use it?

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• How to make G++ preprocessor output a newline in a macro?

  - by Michael Rybak

  Hello, Is there a way in gcc/g++ 4.* to write a macro that expands into several lines? The following code: #define A X \ Y Expands into X Y I need a macro expanding into X Y

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• sizeof float (3.0) vs (3.0f)

  - by kumar

  Hi, What is the difference between sizeof(3.0) and sizeof(3.0f) I was expecting both of them to give the same result (sizeof float)..but its different. In 32 bit machine,gcc compiler, sizeof(3.0f) =4 sizeof(3.0) = 8 Why so?

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• C++ Pointer to a function which takes an instance of a templated class as argument

  - by Nils

  I have trouble compiling a class, which has function pointers as member variables. The pointers are to functions which take an instance of a class as argument. Like template<class T, int N> double (*f)(Vector<T,N> v); I get "error: data member 'f' cannot be a member template" Compiler is gcc 4.2.

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• Is there any small linux distribution which comes with a complete C devleopment environment

  - by hits_lucky

  Hi, I have installed "Damn Small Linux" on my home computer for doing C development in unix. But the distribution doesn't by default come with the C development environment and I am facing some issues when trying to install the gcc. Is there any other small Linux distribution which by default has the required packages for the C development. And also I don't want additional software which takes up lot of space but still would like to have the graphical environment. Thanks

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• When will this condition evaluate to False?

  - by a_m0d

  Under what circumstances will the "False" part of the following code be executed? x = 20; y = -30; if (x > y) { // True part } else { // False part } NB: Language is C, compiler is gcc (although some other compilers may also do the same thing).

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• Run C or C++ file as a script

  - by Brendan Long

  So this is probably a long shot, but is there any way to run a C or C++ file as a script? I tried: #!/usr/bin/gcc main.c -o main; ./main int main(){ return 0; } But it says: ./main.c:1:2: error: invalid preprocessing directive #!

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• #define and how to use them - C++

  - by ML

  Hi All, in a pre-compiled header if I do: #define DS_BUILD #define PGE_BUILD #define DEMO then in source I do: #if (DS_BUILD && DEMO) ---- code--- #elif (PGE_BUILD && DEMO) --- code--- #else --- code --- #endif Do I get an error that states: error: operator '&&' has no right operand I have never seen this before. I am using XCode 3.2, GCC 4.2 on OS X 10.6.3

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• callback pattern

  - by robUK

  Hello, gcc 4.4.3 c89 I am creating a client server application and I will need to implement some callback functions. However, I am not too experienced in callbacks. And I am wondering if anyone knowns some good reference material to follow when designing callbacks. Is there any design patterns that are used for c. I did look at some patterns but there where all c++. Many thanks for any suggestions,

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• Do you know of some performances test of the different ways to get thread local storage in C++?

  - by Vicente Botet Escriba

  I'm doing a library that makes extensive use of a thread local variable. Can you point to some benchmarks that test the performances of the different ways to get thread local variables in C++: C++0x thread_local variables compiler extension (Gcc __thread, ...) boost::threads_specific_ptr pthread Windows ... Does C++0x thread_local performs much better on the compilers providing it?

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• C++ boost function overloaded template

  - by aaa

  I cannot figure out why this segment gives unresolved overloaded function error (gcc version 4.3.4 (Debian 4.3.4-6)): #include <algorithm> #include <boost/function.hpp> int main { typedef boost::function2<const int&, const int&, const int&> max; max m(static_cast<max>(&std::max<int>)); } can you help me, thanks

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• Is a return statement mandatory for C++ functions that do not return void?

  - by AlwaysLearning

  My Herb Schildt book on C++ says: "... In C++, if a function is declared as returning a value, it must return a value." However, if I write a function with a non-void return type and do not return anything, the compiler issues a warning instead of an error: "Control reaches end of non-void function." I use gcc (MinGW) and have set the -pedantic flag.

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• Multiple declarations and definations

  - by Yogesh

  X.c >> ------------------------- int i; main () { fun (); } ------------------------- Y.c >> int i; fun () { } why does these two files compile with no error ? (using GCC) but if i use int i = 10; throws multiple definition error

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• Using .align in inline assemby

  - by tech74

  Hi, I'm using ".align 16 \n\t" in some inline ARM assembly that is implementing some loops to align it on a 16 byte boundary however gcc asm compiler is complaining that alignement is too large i want to implement -falign-loops=16 in asm for a particular loop Thanks

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• C++: combine const with template arguments

  - by awn

  The following example is working when I manualy replace T wirh char *, but why is not working as it is: template <typename T> class A{ public: A(const T _t) { } }; int main(){ const char * c = "asdf"; A<char *> a(c); } When compiling with gcc, I get this error: test.cpp: In function 'int main()': test.cpp:10: error: invalid conversion from 'const char*' to 'char*' test.cpp:10: error: initializing argument 1 of 'A<T>::A(T) [with T = char*]'

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• How should I collect dependencies from Adobe Flex files?

  - by eswald

  I'm looking for a way to collect the dependencies from Flex ActionScript and MXML files. I was hoping that mxmlc could spit them out (like gcc's -M option), but its option list doesn't seem to have anything relevant. I could write a parser, but would prefer not to reinvent the wheel if it has already been done, particularly given the two very different languages involved. Is there a program available to do this for me?

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• Does binding temporary to a reference require a copy constructor in C++?

  - by vitaut

  Consider the following code: class A { A(const A&); public: A() {} }; int main() { const A &a = A(); } This code compiles fine with GCC, but fails to compile with Visual C++ with the following error: test.cc(8) : error C2248: 'A::A' : cannot access private member declared in class 'A' test.cc(2) : see declaration of 'A::A' test.cc(1) : see declaration of 'A' So is it necessary to have a copy constructor accessible when binding a temporary to a reference?

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• Must declare function prototype in C?

  - by Mohit Deshpande

  I am kind of new to C (I have prior Java, C#, and some C++ experience). In C, is it necessary to declare a function prototype or can the code compile without it? Is it good programming practice to do so? Or does it just depend on the compiler? (I am running Ubuntu 9.10 and using the GNU C Compiler, or gcc, under the Code::Blocks IDE)

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• Why would you use the ternary operator without assigning a value for the "true" condition?

  - by RickNotFred

  In the Android open-source qemu code I ran across this line of code: machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */ It this just a confusing way of saying: if (machine->max_cpus) { ; //do nothing } else { machine->max_cpus = 1; } If so, wouldn't it be clearer as: if (machine->max_cpus == 0) machine->max_cpus = 1; Interestingly, this compiles and works fine with gcc, but doesn't compile on http://www.comeaucomputing.com/tryitout/ .

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• How to enable core dump in my Linux C++ program

  - by Alex Farber

  My program is written in C++. compiled with gcc, using -g3 -O0 -ggdb flags. When it crashes, I want to open its core dump. Does it create core dump file, or I need to do something to enable core dump creation, in the program itself, or on computer where it is executed? Where this file is created, and what is its name?

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• Initialising a reference member with itself legal?

  - by epronk

  This was a bug I found in a server application using Valgrind. struct Foo { Foo(const std::string& a) : a_(a_) { } const std::string& a_; }; with gcc -Wall you don't get a warning. Why is this legal code?

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• C: Convert A ? B : C into if (A) B else C

  - by tur1ng

  I was looking for a tool that can convert C code expressions for the form: a = (A) ? B : C; into the 'default' syntax with if/else statements: if (A) a = B else a = C Does someone know a tool that's capable to do such a transformation? I work with GCC 4.4.2 and create a preprocessed file with -E but do not want such structures in it.

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• How to understand such C macro expansion

  - by upton

  A macro definition: #define HTTP_ERRNO_MAP(XX) \ /* No error */ \ XX(OK, "success") \ \ /* Callback-related errors */ \ XX(CB_message_begin, "the on_message_begin callback failed") \ XX(CB_url, "the on_url callback failed") \ /* Define HPE_* values for each errno value above */ #define HTTP_ERRNO_GEN(n, s) HPE_##n, enum http_errno { HTTP_ERRNO_MAP(HTTP_ERRNO_GEN) }; #undef HTTP_ERRNO_GEN After expand it by "gcc -E", enum http_errno { HPE_OK, HPE_CB_message_begin, HPE_CB_url,}; How does the macro expand to the result?

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• Code:Block in window & compiler in linux

  - by ambika

  i have the Code:Block ide in window. my compiler is in linux machine that is GCC. can i write the code in window & compile in linux with the Code:Block ide. if i can , then how ? if not, is there any alternative to do that. thanks in advance for all suggestion.

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• Multiple declarations and definitions

  - by Yogesh

  Content of X.c: int i; main () { fun (); } Content of Y.c: int i; fun () { } Why does these two files compile with no error ? (using GCC) But if i use int i = 10; it prints a multiple definition error.

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