Unable to compile C++ program on a macosx: error: unknown type name 'constexpr'

Question

I am trying to compile the below program. However it's giving me the error: unknown type name 'constexpr' error. What should I do?

Code:

//this is model.cpp. battery.cpp and load.cpp are two secondary files
#include "load.h"
#include "battery.h"

//compile time constants - Settings
constexpr int intervals = 96;
constexpr float e_min = 0.001;
constexpr int max_iters = 100;
constexpr int nloads = 10;
constexpr int nbat = 10;


struct devices
{
    load loadDevices[nloads];
    battery batteryDevices[nbat];
};

int main()
{


    //Initialization of the model
    devices devices1;

    return 0;
}

I get the same error on each line where constexpr is used.

error: unknown type name 'constexpr'
constexpr int intervals = 96;

My C/C++ configurations .json file is as follows:

{
    "configurations": [
        {
            "name": "Mac",
            "includePath": [
                "${workspaceFolder}/**"
            ],
            "defines": [],
            "macFrameworkPath": [],
            "compilerPath": "/usr/local/bin/gcc-11",
            "cStandard": "gnu17",
            "cppStandard": "gnu++17",
            "intelliSenseMode": "macos-gcc-x64"
        }
    ],
    "version": 4
}

System: MACOSX

IDE: VSCode

Compiling on Mac's Terminal using g++

compilation command: g++ model.cpp battery.cpp load.cpp

Hi Jarod,I am a beginner programmer in C++. Are you saying I should type g++ -std=c++11 model.cpp battery.cpp load.cpp in the mac terminal to compile? — adityaishwar1994
– adityaishwar1994, Commented Aug 5, 2021 at 9:42
Yes (even use more recent standard if possible). Giving version of gcc might help else. — Jarod42
– Jarod42, Commented Aug 5, 2021 at 9:44
Okay. The constexpr error goes away when I compile with -std=c++11 and onwards. However now post compiling I get the Undefined symbols for architecture x86_64: error for the method "optAlg::bufferPlanning(float*, float, float, int, float*, float*, float, float, float*, float*, bool, float*, int, float*, float*)", referenced from: battery::plan(float*) in battery-011c04.o — adityaishwar1994
– adityaishwar1994, Commented Aug 5, 2021 at 9:50

koder · Accepted Answer · 2021-08-05 09:46:03Z

1

That json config is not used if you are calling the compiler directly from the command line. In that case you have to specify every option yourself:

g++ -std=gnu++17 -Wall -Werror model.cpp battery.cpp load.cpp

I just added -Wall -Werror for good measure, you should never compile your code without them.

Without the -std option the compiler uses an older version of the C++ standard that does not have constexpr.

answered Aug 5, 2021 at 9:46

koder

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