Expected primary-expression before `alignas(X)`?

Question

I want to test the use of alignas(), so I write down these code:

#include <vector>

using namespace std;

template<typename X> 
void user(const vector<X>& vx)
{
    constexpr int bufmax = 1024;
    alignas(X) buffer[bufmax];

    const int max = min(vx.size(), bufmax / sizeof(X));
    uninitialized_copy(vx.begin(), vx.begin()+max, buffer);
}

However, when I compile it with g++, the compiler outputs an error: "expected primary-expression before alignas(X)". Who could explain this? I don't know the exact usages of alignas().

I guess it is `X`. These codes are come from the book The C++ Programming Language 4th edition. — Shangchih Huang, Mar 15 '17 at 04:37
I might have phrased this incorrectly. What part of line 9 specifies type of `buffer`? Also mandatory: What version of GCC do you use? Did you turn on C++11 features? — Revolver_Ocelot, Mar 15 '17 at 04:40
The version of GCC is 5.3.0, and yes, I turn on c++11 features. In fact, I am not sure the type of `buffer`, Is `alignas(X)` a declaration? — Shangchih Huang, Mar 15 '17 at 04:46

score 0 · Answer 1 · answered Mar 15 '17 at 04:53

0

alignas(X) is an aligment specifier, which is applied to type. Your error happens because you didn't specified a type here.

Judging from the rest of the code, you probably want to create aligned buffer of characters: alignas(X) char buffer[bufmax];

answered Mar 15 '17 at 04:53

Revolver_Ocelot

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score 0 · Answer 2 · edited Feb 07 '20 at 13:15

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add below data type to your code

char alignas(X) buffer[bufmax];

alignas(X) char buffer[bufmax];

I also bumped on the same while reading the book.

edited Feb 07 '20 at 13:15

Omari Victor Omosa

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answered Dec 26 '17 at 13:15

Victor Mwenda

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Expected primary-expression before `alignas(X)`?

2 Answers2