This question states that main can be implementation defined with some restrictions.
So, I wrote the following C++ code to try out the following signature of main:
main.h
class MyClass {
private:
int i;
public:
MyClass();
inline int geti() {
return i;
}
inline void seti(int i) {
this->i = i;
}
~MyClass();
};
MyClass::MyClass() {
this->i = 2;
}
MyClass::~MyClass() {
}
main.c++
#include <iostream>
#include "main.h"
int main(MyClass myClass) {
std::cout << myClass.geti() << std::endl;
return 0;
}
Which Gives the following results:
The command g++ -o main main.c++ -O3 compiles successfully with warnings:
main.c++:5:5: warning: first argument of ‘int main(MyClass)’ should be ‘int’ [-Wmain]
5 | int main(MyClass myClass) {
| ^~~~
main.c++:5:5: warning: ‘int main(MyClass)’ takes only zero or two arguments [-Wmain]
The command clang++ -o main main.c++ -std=c++14 gives the error:
main.c++:5:5: error: first parameter of 'main' (argument count) must be of type 'int'
int main(MyClass myClass) {
^
1 error generated.
the main file generated by g++ gives SIGSEGV (why though?)
So, if main can be implementation defined, why does clang give an error while g++ generated file give SIGSEGV?
I also went further and created a different code so that I will be able to pass a MyClass object to main.c++ as follows:
#include <iostream>
#include "main.h"
#include <unistd.h>
int main() {
MyClass myClass;
execve("./main",myClass,NULL);
return 0;
}
However, as execve takes the second parameter to be a char* const *, it does not compile. How do I pass the myClass object to the main file generated by g++?
The command g++ -o main main.c++ -O3 compiles successfully with warnings
This is not successful compilation. You should always use -Werror. If you fail to do so and then decide to ignore the warning and proceed with running the program, it's your own responsibility. You better know full well what you are doing. See this for more information.
the main file generated by g++ gives SIGSEGV (why though?)
The compiler has warned you. It is in your best interest to listen to it. If things go boom, chances are, that's because you have ignored warnings.
why does clang give an error while g++ generated file give SIGSEGV?
The program is not a valid C++ program. There is no meaningful difference between a warning and an error.
How do I pass the myClass object to the main file generated by g++?
You cannot. main must have a form equivalent to one of these two:
int main()
int main(int argc, char* argv[])
(Optional reading in italics) Other forms of main are implementation-defined. This means your implementation needs to support them in a documented way. Unless you have read documentation for your implementation and found that it supports the form of main you want, there's no way to do that.
Other than having an implementation-defined main, the only way a program can get hold of an object of a class type is by constructing that object.
You are close. You have identified your primary issue attempting to pass as a parameter to main() -- that won't work. The declaration for main() is defined by the standard and you are limited to passing string values (nul-terminated character arrays... C-Strings) in as arguments.
In your case you need to create an instance of your class within main(), e.g.
#include <iostream>
#include "main.h"
int main() {
MyClass myClass;
std::cout << myClass.geti() << std::endl;
return 0;
}
Your main.h header has a variable shadowing problem where at line 10:
inline void seti(int i) {
int i shadows a prior declaration at line 3, e.g. int i; (though the consequence would be unlikely to matter). Just replace the variable name in the second declaration with j (or whatever you like). Your code will compile without warning, e.g.
class MyClass {
private:
int i;
public:
MyClass();
inline int geti() {
return i;
}
inline void seti(int j) {
this->i = j;
}
~MyClass();
};
MyClass::MyClass() {
this->i = 2;
}
MyClass::~MyClass() {
}
Example Use/Output
$ ./bin/main
2
You can also call your seti() function to update the private variable in your class, e.g.
myClass.seti(5);
std::cout << myClass.geti() << std::endl;
Which would now output 5.
Let me know if you have further questions.
The sample below shows reading an uninitialized field a.i1 in two ways. The first call does not compile. However, calling a.donothing() that does not do anything, the original call compiles fine and prints the default value 0. Why is this inconsistency?
I am using Visual Studio Community 2015, the compiler output is as follows:
Severity Code Description Project File Line Suppression State
Error C4700 uninitialized local variable 'a' used
class A {
public:
int i1;
void donothing() {}
};
int main() {
A a;
cout << "main1: " << a.i1 << endl; // compile fails
a.donothing();
cout << "main2: " << a.i1 << endl; // prints 0 (default)
return 0;
}
Compiler is doing what it ought to do. You can fix it like this (as one solution out of many):
class A {
public:
A(int i = 0) : i1(i) {}
int i1;
void donothing() {}
};
In both cases a warning must be issued, at most. The fact that calling donothing cancels the error is a clear indication that this is a bug. You can report it at Microsoft Connect.
A simple workaround for this problem is to change the declaration to A a{};.
You can test your code on different compilers at Compiler Explorer.
[EDIT] The warning message C4700 is treated as an error if Security Development Lifecycle is turned on (/sdl).
Depends on the compiler, the compiler should supply a default constructor that will initialize your members with a default value. But this behavior is not dependable. Since C++11 you can say ClassName()=default; The best practice is to prove your own default constructor.
Your code never had any compiler errors with g++ 5.4.0
#include <iostream>
using namespace std;
class A {
public:
//A() : i1(0) { } // compiler will provide this if you don't write anything
// since C++ 11 you can also say A() = default;
A() = default;
int i1;
void donothing() {}
void writeMember() const { cout << "i1 value: " << i1 << endl; }
};
// better provide a signature for the main function
int main(int argc, char* argv[]) {
A a;
a.writeMember();
cout << "main1: " << a.i1 << endl; // compile fails
a.donothing();
cout << "main2: " << a.i1 << endl; // prints 0 (default)
return 0;
}
To compile the above code stored in testclass.cpp
g++ -std=c++11 -o testclass testclass.cpp
By using the C++11 default I got
i1 value: 4196976
main1: 4196976
main2: 4196976
If you comment out A()=default; this will rely on the compiler provided initializer, or the compiler may be lazy and not doing anything for performance reasons. You get
i1 value: 4196944
main1: 4196944
main2: 4196944
If you uncomment the line after public: you should consistently get 0
This illustrates the importance of adhering to good conventions of alway provide your own default constructor. The compiler maybe doing the right thing by not assigning any particular value to your member because you may assign another value in the next operation. This can save one operation. The member will be simply allocated on the stack, in that case the member got a random value. If you run this code on a different computer you will for sure get a different value.
Can not understand weird program behavior - hopefully someone can explain.
dummy.h:
#ifndef DUMMY_H
#define DUMMY_H
#include <iostream>
class Dummy
{
int val;
public:
int Init(int new_val)
{
return val = new_val;
}
int Get()
{
return val;
}
Dummy():
val(-1)
{
std::cout << "constructed" << std::endl;
}
~Dummy()
{
std::cout << "deconstructed" << std::endl;
}
};
#endif /*DUMMY_H*/
header.h:
#include "dummy.h"
extern Dummy dummy;
dummy.cpp:
#include "dummy.h"
Dummy dummy;
main.cpp:
#include <iostream>
#include "header.h"
int res1 = dummy.Init(2);
int res2 = dummy.Get();
int main()
{
std::cout << res1 << std::endl;
std::cout << res2 << std::endl;
std::cout << dummy.Get() << std::endl;
return 0;
}
compiling: g++ -Wall -Wextra main.cpp dummy.cpp
output:
constructed
2
2
-1
deconstructed
why the 2nd Get(), called in main() function returns -1? Why the assigned value disappears from the dummy instance and no deconstructor was called. How it becomes -1?
upd: added debug info to Init() and Get():
new output:
init
get
constructed
2
2
get
-1
deconstructed
upd2: funny fact - separate compiling and linking object files in one executable changed the situation:
g++ -c dummy.cpp
g++ -c main.cpp
g++ dummy.o main.o
./a.out
constructed
init
get
2
2
get
2
deconstructed
but IT IS A TRAP!
The order of initialization of global variables in different translation units is unspecified. Either dummy or the two global ints in main.cpp could be constructed first.
In your case, it's res1 and res2 that got initialized first, so you invoked undefined behaviour by calling member functions on object that wasn't yet constructed.
To fix it, try this:
header.h:
#include "dummy.h"
Dummy& getDummy();
dummy.cpp:
#include "dummy.h"
Dummy& getDummy()
{
static Dummy dummy; // gets initialized at first call
// and persists for the duration of the program
return dummy;
}
main.cpp
int res1 = getDummy().Init(2);
int res2 = getDummy().Get();
If it helps understanding, add some debug cout statements to Get and Init, too.
What you experienced is also known as static initialization order fiasco.
So, what happens is that int res1 = dummy.Init(2); and int res2 = dummy.Get(); is run BEFORE dummy has been initialized. Then, before main is entered, dummy gets constructed and has the value -1 stored in val.
Yyou can possibly change this in this particular case, for this version of the compiler, by rearranging your object files to g++ -o myprog dummy.o main.o instead of g++ -o myprog main.o dummy.o - but that will not GUARANTEE to fix the problem, and in a future version of compiler/linker, the result may well alter again. The C++ standard gives no guarantee at all - I'm just suggesting the "order may matter" from what I've personally seen. And since there is no particular requirement for these things, the compiler vendor is allowed to change it in any which way they like at any time.
C++ is a static, compiled language, templates are resolved during compile time and so on...
But is it possible to create a function during runtime, that is not described in the source code and has not been converted to machine language during compilation, so that a user can throw at it data that has not been anticipated in the source?
I am aware this cannot happen in a straightforward way, but surely it must be possible, there are plenty of programing languages that are not compiled and create that sort of stuff dynamically that are implemented in either C or C++.
Maybe if factories for all primitive types are created, along with suitable data structures to organize them into more complex objects such as user types and functions, this is achievable?
Any info on the subject as well as pointers to online materials are welcome. Thanks!
EDIT: I am aware it is possible, it is more like I am interested in implementation details :)
Yes, of course, without any tools mentioned in the other answers, but simply using the C++ compiler.
just follow these steps from within your C++ program (on linux, but must be similar on other OS)
write a C++ program into a file (e.g. in /tmp/prog.cc), using an ofstream
compile the program via system("c++ /tmp/prog.cc -o /tmp/prog.so -shared -fPIC");
load the program dynamically, e.g. using dlopen()
You can also just give the bytecode directly to a function and just pass it casted as the function type as demonstrated below.
e.g.
byte[3] func = { 0x90, 0x0f, 0x1 }
*reinterpret_cast<void**>(&func)()
Yes, JIT compilers do it all the time. They allocate a piece of memory that has been given special execution rights by the OS, then fill it with code and cast the pointer to a function pointer and execute it. Pretty simple.
EDIT: Here's an example on how to do it in Linux: http://burnttoys.blogspot.de/2011/04/how-to-allocate-executable-memory-on.html
Below an example for C++ runtime compilation based on the method mentioned before (write code to output file, compile via system(), load via dlopen() and dlsym()). See also the example in a related question. The difference here is that it dynamically compiles a class rather than a function. This is achieved by adding a C-style maker() function to the code to be compiled dynamically. References:
https://www.linuxjournal.com/article/3687
http://www.tldp.org/HOWTO/C++-dlopen/thesolution.html
The example only works under Linux (Windows has LoadLibrary and GetProcAddress functions instead), and requires the identical compiler to be available on the target machine.
baseclass.h
#ifndef BASECLASS_H
#define BASECLASS_H
class A
{
protected:
double m_input; // or use a pointer to a larger input object
public:
virtual double f(double x) const = 0;
void init(double input) { m_input=input; }
virtual ~A() {};
};
#endif /* BASECLASS_H */
main.cpp
#include "baseclass.h"
#include <cstdlib> // EXIT_FAILURE, etc
#include <string>
#include <iostream>
#include <fstream>
#include <dlfcn.h> // dynamic library loading, dlopen() etc
#include <memory> // std::shared_ptr
// compile code, instantiate class and return pointer to base class
// https://www.linuxjournal.com/article/3687
// http://www.tldp.org/HOWTO/C++-dlopen/thesolution.html
// https://stackoverflow.com/questions/11016078/
// https://stackoverflow.com/questions/10564670/
std::shared_ptr<A> compile(const std::string& code)
{
// temporary cpp/library output files
std::string outpath="/tmp";
std::string headerfile="baseclass.h";
std::string cppfile=outpath+"/runtimecode.cpp";
std::string libfile=outpath+"/runtimecode.so";
std::string logfile=outpath+"/runtimecode.log";
std::ofstream out(cppfile.c_str(), std::ofstream::out);
// copy required header file to outpath
std::string cp_cmd="cp " + headerfile + " " + outpath;
system(cp_cmd.c_str());
// add necessary header to the code
std::string newcode = "#include \"" + headerfile + "\"\n\n"
+ code + "\n\n"
"extern \"C\" {\n"
"A* maker()\n"
"{\n"
" return (A*) new B(); \n"
"}\n"
"} // extern C\n";
// output code to file
if(out.bad()) {
std::cout << "cannot open " << cppfile << std::endl;
exit(EXIT_FAILURE);
}
out << newcode;
out.flush();
out.close();
// compile the code
std::string cmd = "g++ -Wall -Wextra " + cppfile + " -o " + libfile
+ " -O2 -shared -fPIC &> " + logfile;
int ret = system(cmd.c_str());
if(WEXITSTATUS(ret) != EXIT_SUCCESS) {
std::cout << "compilation failed, see " << logfile << std::endl;
exit(EXIT_FAILURE);
}
// load dynamic library
void* dynlib = dlopen (libfile.c_str(), RTLD_LAZY);
if(!dynlib) {
std::cerr << "error loading library:\n" << dlerror() << std::endl;
exit(EXIT_FAILURE);
}
// loading symbol from library and assign to pointer
// (to be cast to function pointer later)
void* create = dlsym(dynlib, "maker");
const char* dlsym_error=dlerror();
if(dlsym_error != NULL) {
std::cerr << "error loading symbol:\n" << dlsym_error << std::endl;
exit(EXIT_FAILURE);
}
// execute "create" function
// (casting to function pointer first)
// https://stackoverflow.com/questions/8245880/
A* a = reinterpret_cast<A*(*)()> (create)();
// cannot close dynamic lib here, because all functions of the class
// object will still refer to the library code
// dlclose(dynlib);
return std::shared_ptr<A>(a);
}
int main(int argc, char** argv)
{
double input=2.0;
double x=5.1;
// code to be compiled at run-time
// class needs to be called B and derived from A
std::string code = "class B : public A {\n"
" double f(double x) const \n"
" {\n"
" return m_input*x;\n"
" }\n"
"};";
std::cout << "compiling.." << std::endl;
std::shared_ptr<A> a = compile(code);
a->init(input);
std::cout << "f(" << x << ") = " << a->f(x) << std::endl;
return EXIT_SUCCESS;
}
output
$ g++ -Wall -std=c++11 -O2 -c main.cpp -o main.o # c++11 required for std::shared_ptr
$ g++ -ldl main.o -o main
$ ./main
compiling..
f(5.1) = 10.2
Have a look at libtcc; it is simple, fast, reliable and suits your need. I use it whenever I need to compile C functions "on the fly".
In the archive, you will find the file examples/libtcc_test.c, which can give you a good head start.
This little tutorial might also help you: http://blog.mister-muffin.de/2011/10/22/discovering-tcc/
#include <stdlib.h>
#include <stdio.h>
#include "libtcc.h"
int add(int a, int b) { return a + b; }
char my_program[] =
"int fib(int n) {\n"
" if (n <= 2) return 1;\n"
" else return fib(n-1) + fib(n-2);\n"
"}\n"
"int foobar(int n) {\n"
" printf(\"fib(%d) = %d\\n\", n, fib(n));\n"
" printf(\"add(%d, %d) = %d\\n\", n, 2 * n, add(n, 2 * n));\n"
" return 1337;\n"
"}\n";
int main(int argc, char **argv)
{
TCCState *s;
int (*foobar_func)(int);
void *mem;
s = tcc_new();
tcc_set_output_type(s, TCC_OUTPUT_MEMORY);
tcc_compile_string(s, my_program);
tcc_add_symbol(s, "add", add);
mem = malloc(tcc_relocate(s, NULL));
tcc_relocate(s, mem);
foobar_func = tcc_get_symbol(s, "foobar");
tcc_delete(s);
printf("foobar returned: %d\n", foobar_func(32));
free(mem);
return 0;
}
Ask questions in the comments if you meet any problems using the library!
In addition to simply using an embedded scripting language (Lua is great for embedding) or writing your own compiler for C++ to use at runtime, if you really want to use C++ you can just use an existing compiler.
For example Clang is a C++ compiler built as libraries that could be easily embedded in another program. It was designed to be used from programs like IDEs that need to analyze and manipulate C++ source in various ways, but using the LLVM compiler infrasructure as a backend it also has the ability to generate code at runtime and hand you a function pointer that you can call to run the generated code.
Clang
LLVM
Essentially you will need to write a C++ compiler within your program (not a trivial task), and do the same thing JIT compilers do to run the code. You were actually 90% of the way there with this paragraph:
I am aware this cannot happen in a straightforward way, but surely it
must be possible, there are plenty of programing languages that are
not compiled and create that sort of stuff dynamically that are
implemented in either C or C++.
Exactly--those programs carry the interpreter with them. You run a python program by saying python MyProgram.py--python is the compiled C code that has the ability to interpret and run your program on the fly. You would need do something along those lines, but by using a C++ compiler.
If you need dynamic functions that badly, use a different language :)
A typical approach for this is to combine a C++ (or whatever it's written on) project with scripting language.
Lua is one of the top favorites, since it's well documented, small, and has bindings for a lot of languages.
But if you are not looking into that direction, perhaps you could think of making a use of dynamic libraries?
Yes - you can write a compiler for C++, in C++, with some extra features - write your own functions, compile and run automatically (or not)...
Have a look into ExpressionTrees in .NET - I think this is basically what you want to achieve. Create a tree of subexpressions and then evaluate them. In an object-oriented fashion, each node in the might know how to evaluate itself, by recursion into its subnodes. Your visual language would then create this tree and you can write a simple interpreter to execute it.
Also, check out Ptolemy II, as an example in Java on how such a visual programming language can be written.
You could take a look at Runtime Compiled C++ (or see RCC++ blog and videos), or perhaps try one of its alternatives.
Expanding on Jay's answer using opcodes, the below works on Linux.
Learn opcodes from your compiler:
write own myfunc.cpp, e.g.
double f(double x) { return x*x; }
compile with
$ g++ -O2 -c myfunc.cpp
disassemble function f
$ gdb -batch -ex "file ./myfunc.o" -ex "set disassembly-flavor intel" -ex "disassemble/rs f"
Dump of assembler code for function _Z1fd:
0x0000000000000000 <+0>: f2 0f 59 c0 mulsd xmm0,xmm0
0x0000000000000004 <+4>: c3 ret
End of assembler dump.
This means the function x*x in assembly is mulsd xmm0,xmm0, ret and in machine code f2 0f 59 c0 c3.
Write your own function in machine code:
opcode.cpp
#include <cstdlib> // EXIT_FAILURE etc
#include <cstdio> // printf(), fopen() etc
#include <cstring> // memcpy()
#include <sys/mman.h> // mmap()
// allocate memory and fill it with machine code instructions
// returns pointer to memory location and length in bytes
void* gencode(size_t& length)
{
// machine code
unsigned char opcode[] = {
0xf2, 0x0f, 0x59, 0xc0, // mulsd xmm0,xmm0
0xc3 // ret
};
// allocate memory which allows code execution
// https://en.wikipedia.org/wiki/NX_bit
void* buf = mmap(NULL,sizeof(opcode),PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANON,-1,0);
// copy machine code to executable memory location
memcpy(buf, opcode, sizeof(opcode));
// return: pointer to memory location with executable code
length = sizeof(opcode);
return buf;
}
// print the disassemby of buf
void print_asm(const void* buf, size_t length)
{
FILE* fp = fopen("/tmp/opcode.bin", "w");
if(fp!=NULL) {
fwrite(buf, length, 1, fp);
fclose(fp);
}
system("objdump -D -M intel -b binary -mi386 /tmp/opcode.bin");
}
int main(int, char**)
{
// generate machine code and point myfunc() to it
size_t length;
void* code=gencode(length);
double (*myfunc)(double); // function pointer
myfunc = reinterpret_cast<double(*)(double)>(code);
double x=1.5;
printf("f(%f)=%f\n", x,myfunc(x));
print_asm(code,length); // for debugging
return EXIT_SUCCESS;
}
compile and run
$ g++ -O2 opcode.cpp -o opcode
$ ./opcode
f(1.500000)=2.250000
/tmp/opcode.bin: file format binary
Disassembly of section .data:
00000000 <.data>:
0: f2 0f 59 c0 mulsd xmm0,xmm0
4: c3 ret
The simplest solution available, if you're not looking for performance is to embed a scripting language interpreter, e.g. for Lua or Python.
It worked for me like this. You have to use the -fpermissive flag.
I am using CodeBlocks 17.12.
#include <cstddef>
using namespace std;
int main()
{
char func[] = {'\x90', '\x0f', '\x1'};
void (*func2)() = reinterpret_cast<void*>(&func);
func2();
return 0;
}
I want to implement a function tracer, which would trace how much time a function is taking to execute. I have following class for the same:-
class FuncTracer
{
public:
FuncTracer(LPCTSTR strFuncName_in)
{
m_strFuncName[0] = _T('\0');
if( strFuncName_in ||
_T('\0') != strFuncName_in[0])
{
_tcscpy(m_strFuncName,strFuncName_in);
TCHAR strLog[MAX_PATH];
_stprintf(strLog,_T("Entering Func:- <%s>"),m_strFuncName);
LOG(strLog)
m_dwEnterTime = GetTickCount();
}
}
~FuncTracer()
{
TCHAR strLog[MAX_PATH];
_stprintf(strLog,_T("Leaving Func:- <%s>, Time inside the func <%d> ms"),m_strFuncName, GetTickCount()-m_dwEnterTime);
LOG(strLog)
}
private:
TCHAR m_strFuncName[MAX_PATH];
DWORD m_dwEnterTime;
};
void TestClass::TestFunction()
{
// I want to avoid writing the function name maually..
// Is there any macro (__LINE__)or some other way to
// get the function name inside a function ??
FuncTracer(_T("TestClass::TestFunction"));
/*
* Rest of the function code.
*/
}
I want to know if there is any way to get the name of the function from inside of a function? Basically I want the users of my class to simply create an object the same. They may not pass the function name.
C99 has __func__, but for C++ this will be compiler specific. On the plus side, some of the compiler-specific versions provide additional type information, which is particularly nice when you're tracing inside a templatized function/class.
MSVC: __FUNCTION__, __FUNCDNAME__, __FUNCSIG__
GCC: __func__, __FUNCTION__, __PRETTY_FUNCTION__
Boost library has defined macro BOOST_CURRENT_FUNCTION for most C++ compilers in header boost/current_function.hpp. If the compiler is too old to support this, the result will be "(unknown)".
VC++ has
__FUNCTION__ for undecorated names
and
__FUNCDNAME__ for decorated names
And you can write a macro that will itself allocate an object and pass the name-yelding macro inside the constructor. Smth like
#define ALLOC_LOGGER FuncTracer ____tracer( __FUNCTION__ );
C++20 std::source_location::function_name
main.cpp
#include <iostream>
#include <string_view>
#include <source_location>
void log(std::string_view message,
const std::source_location& location = std::source_location::current()
) {
std::cout << "info:"
<< location.file_name() << ":"
<< location.line() << ":"
<< location.function_name() << " "
<< message << '\n';
}
int f(int i) {
log("Hello world!"); // Line 16
return i + 1;
}
int f(double i) {
log("Hello world!"); // Line 21
return i + 1.0;
}
int main() {
f(1);
f(1.0);
}
Compile and run:
g++ -ggdb3 -O0 -std=c++20 -Wall -Wextra -pedantic -o source_location.out source_location.cpp
./source_location.out
Output:
info:source_location.cpp:16:int f(int) Hello world!
info:source_location.cpp:21:int f(double) Hello world!
so note how the call preserves caller information, so we see the desired main call location instead of log.
I have covered the relevant standards in a bit more detail at: What's the difference between __PRETTY_FUNCTION__, __FUNCTION__, __func__?
Tested on Ubuntu 22.04, GCC 11.3.
I was going to say I didn't know of any such thing but then I saw the other answers...
It might interest you to know that an execution profiler (like gprof) does exactly what you're asking about - it tracks the amount of time spent executing each function. A profiler basically works by recording the instruction pointer (IP), the address of the currently executing instruction, every 10ms or so. After the program is done running, you invoke a postprocessor that examines the list of IPs and the program, and converts those addresses into function names. So I'd suggest just using the instruction pointer, rather than the function name, both because it's easier to code and because it's more efficient to work with a single number than with a string.