#include "connection.h"

void setup() {
  Serial.begin(115200);
  Funcs::Connect conn;
  conn.connectWifi();
}

void loop() {

}

then connection.h file:

// connection.h

namespace Funcs
{
  class Connect 
  {
    public:
      void connectWifi();
  };
}

and then connection.cpp

#include <Arduino.h>
#include <ESP8266WiFi.h>
#include "connection.h"
const char* ssid     = "ssid";
const char* password = "password";


namespace Funcs
{

  void connectWifi() {
    // Connect to WiFi network
    Serial.println();
    Serial.println();
    Serial.print("Connecting to ");
    Serial.println(ssid);
  
    WiFi.begin(ssid, password);
  
    while (WiFi.status() != WL_CONNECTED) {
      delay(10);
      Serial.print(".");
    }
    Serial.println("");
    Serial.print("WiFi connected with ip ");
    Serial.println(WiFi.localIP());
  
  }
}

On compiling I get this error:

collect2: error: ld returned 1 exit status
exit status 1
Error compiling for board WeMos D1 R1.

#include "connection.h"

void setup() {
  Serial.begin(115200);
  Funcs::Connect conn;
  conn.connectWifi();
}

void loop() {

}

then connection.h file:

// connection.h

namespace Funcs
{
  class Connect 
  {
    public:
      void connectWifi();
  };
}

and then connection.cpp

#include <Arduino.h>
#include <ESP8266WiFi.h>
#include "connection.h"
const char* ssid     = "ssid";
const char* password = "password";


namespace Funcs
{

  void connectWifi() {
    // Connect to WiFi network
    Serial.println();
    Serial.println();
    Serial.print("Connecting to ");
    Serial.println(ssid);
  
    WiFi.begin(ssid, password);
  
    while (WiFi.status() != WL_CONNECTED) {
      delay(10);
      Serial.print(".");
    }
    Serial.println("");
    Serial.print("WiFi connected with ip ");
    Serial.println(WiFi.localIP());
  
  }
}

On compiling I get this error:

/Library/Arduino15/packages/esp8266/hardware/esp8266/2.5.2/cores/esp8266/HardwareSerial.h:76: 
undefined reference to `Funcs::Connect::connectWifi()'

collect2: error: ld returned 1 exit status

collect2: error: ld returned 1 exit status
exit status 1
Error compiling for board WeMos D1 R1.

But I do not have Funcs::Connect::connectWifi() in my code, it was in old code.