Access auto declared array - c++

auto messwerte2 = { 3.5, 7.3, 4.9, 8.3, 4.4, 5.3, 3.8, 7.5 };
Which possibilities exist to access a single value explicitly of this array-like looking structure which as I was informed is actually a std::initializer_list ?


Which possibilities exist to access a single value explicitly of this array?
It's not an array, but is deduced to a std::initializer_list<double> which can be accessed through an iterator or a range based loop:
#include <iostream>
auto messwerte2 = { 3.5, 7.3, 4.9, 8.3, 4.4, 5.3, 3.8, 7.5 };
int main() {
for(auto x : messwerte2) {
std::cout << x << std::endl;
}
}
Live Demo
To make it an array use an explicit array declaration:
double messwerte2[] = { 3.5, 7.3, 4.9, 8.3, 4.4, 5.3, 3.8, 7.5 };


auto messwerte2 = { 3.5, 7.3, 4.9, 8.3, 4.4, 5.3, 3.8, 7.5 };
doesn't declare an array. It declares a std::initializer_list<double>
To declare an array, use:
double messwerte2[] = { 3.5, 7.3, 4.9, 8.3, 4.4, 5.3, 3.8, 7.5 };
Then you can use the regular array indexing syntax.

Related

clang standard library bug or c++ undefined behavior?

Does the following C++ program contain any undefined behavior?
int
main()
{
struct entry
{
uint32_t hash;
uint32_t idx;
};
entry arr[31] = {
{ 7978558, 0}, { 9241630, 1}, { 65706826, 2},
{ 639636154, 3}, {1033996244, 4}, {1225598536, 5},
{1231940272, 6}, {1252372402, 7}, {2019146042, 8},
{1520971906, 9}, {1532931792, 10}, {1818609302, 11},
{1971583702, 12}, {2116478830, 13}, { 883396844, 14},
{1942092984, 15}, {1274626222, 16}, { 333950222, 17},
{1265547464, 18}, { 965867746, 19}, {1471376532, 20},
{ 398997278, 21}, {1414926784, 22}, {1831587680, 23},
{ 813761492, 24}, { 138146428, 25}, { 337412092, 26},
{ 329155246, 27}, { 21320082, 28}, {1751867558, 29},
{1155173784, 30},
};
std::sort(std::begin(arr), std::end(arr),
[](entry a, entry b) { return a.hash <= b.hash; });
}
When I compile with gnu c++ compiler or any clang/llvm after 12.0.0, the program works fine. However, when I compiled it with clang version 12.0.0 (the default compiler shipped on my Mac laptop), it crashed inside of std::sort() as following:
$ g++ --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.0 (clang-1200.0.32.2)
Target: x86_64-apple-darwin21.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
$ g++ -g -std=c++11 bug.cc
$ ./a.out
Segmentation fault: 11
Also if I change it to use std::vector instead of fixed size array. The std::sort() will never return when compiled with clang 12.0.0

Yes, the comparator is not a strict weak ordering which violates the preconditions of std::sort, resulting in undefined behavior.
For two arguments a and b (possibly identical), a strict weak ordering comp should never evaluate both comp(a,b) and comp(b,a) to true. In other words, it should model the behavior of the built-in <, not <=.
So in your code it should be <, not <=, to make it a strict weak ordering.

Initializer list of std::pair<vector<int>, double>

Hi I am working through some C++ issues and I'm not familiar with the language.
I am trying to initialize a std::pair, double> with this syntax:
std::pair<std::vector<int>, double> output = { {}, 0.0f };
gcc 5.4.0 on Ubuntu 16.04 generates this error:
no known conversion for argument 1 from ‘std::pair<std::vector<int>, double>’ to ‘std::initializer_list<int>
The same error happens if I use this syntax:
std::pair<std::vector<int>, double> output{{}, 0.0f };
What is the issue?

You are using an extended initializer list (std::initializer_list) which is available since C++11.
For the gcc 5.4.0 compiler, you need to compile it with C++11 flag:
$ g++ main.cpp -std=c++11
https://gcc.godbolt.org/z/SHzREE

Does clang on Mac not support uniform initialization?

Is clang on Mac not support uniform initialization?
I tried compiling following code, But compiler raise a error.
#include <iostream>
#include <vector>
#include <string>
int main() {
std::vector<int> v = {3, 1, 9, 4};
std::cout << v[1] << std::endl;
}
Error:
vector.cpp:9:22: error: non-aggregate type 'std::vector<int>' cannot be initialized with an initializer list
std::vector<int> v = {3, 1, 9, 4};
^ ~~~~~~~~~~~~
1 error generated.
OS: macOS 10.12.4
compiler version:
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

I suspect you are not compiling the code as C++11/14 (-std=c++11 or -std=c++14)? If not; do that. Clang does support what you are trying to do.

CUDA 6.5 with g++ does not support c++11?

I read it here that CUDA 6.5 has started support for C++11 :
https://groups.google.com/forum/#!topic/thrust-users/R37GIkMG4tk
But when I compile an example code below, I got
$ nvcc -std=c++11 cu-gcc11.cu -o test
nvcc warning : The -c++11 flag is not supported with the configured host compiler. Flag will be ignored.
cu-gcc11.cu(7): error: explicit type is missing ("int" assumed)
My setting : CUDA 6.5, g++ 4.5, ubuntu 12.04
Codes :
#include <cuda.h>
#include <iostream>
__host__ void test() {
float a = 12.;
double b = 3.;
auto c = a * b;
std::cout << c << std::endl;
}
int main()
{
test();
return 0;
}

C++11 support in nvcc is experimental at this time. In order to properly use it you will need an appropriate configuration. This is not documented anywhere AFAIK, but you should have good results with either Fedora 20 or Ubuntu 14.04, both of which are supported configs for cuda 6.5 and include GCC 4.8.x.
In your case your GCC version is just too old.

I don't think -std=c++11 was available in GCC 4.5. Try -std=c++0x.

can I use auto with g++ 4.4?

I can specify -std=c++0x for compilation with my g++ 4.4, and initializer lists are correct, I may use them (in c++98 I can't) but still get errors when try use auto keyword:
std::list< std::vector<int> > li2;
li2.push_back({1, 2, 3}); //push_back vector
li2.push_back({4, 2, 6}); //again, vector implicitly
for (auto& vv : li2) {
for (auto &i : v)
printf("element: %d\n", 8);
}
so I assume I can't use C++11 functionallities with g++4.4. I have 4.4 because of compatibility with CUDA.

This link shows you the different C++11 features supported by GCC. auto appeared in GCC 4.4.
Your real problem is probably that the ranged-based for loop appeared only in GCC 4.6.