constructor problem

[wchpikus]

i have one question..
how to use constructor in other included files c, when i like to use function u8g2?
in main.ino i use:
U8G2_SSD1322_NHD_256X64_F_4W_HW_SPI u8g2(U8G2_R0, /* cs=/ 15, / dc=/ 4, / reset=/ 5); //dc=/ 4 -> dc=*/ 4

and any function in this file work fine, but when i like to use any function in other c file, i got e.g
display.cpp:75:1: error: 'u8g2' was not declared in this scope..
Only then i add in function constructor again, it works.